The first thing I learned about politic is that power lies with the people where people think it lies. You can only force someone to follow your orders if the other person thinks you have the power to either force him or make him a good offer to convince him, both would use power. It's really a catch 22. That usually means the person has the backing of other people, say the military, who can force you to obey their leader. But it can also mean the person has for example a powerful weapon in his hands, or a precious painting that you want. Ultimately it is not a title that gives people influence, but other people believing that they have power.
The same is true for a Constitution. Sure, there is a piece of paper somewhere, proclaiming that "Congress has the sole power to declare war", but that makes no difference unless people in power actually follow that rule.
Any political system has a constitution, whether that's explicitely or just implied. The word "constitution" has taken on a new meaning since the first written Constitutions showed up, but when you think about it, "constitution" really just means "the state of affairs the country is in". There is always a de facto Constitution, but by declaring something to be the "Constitution" of the lans, all you're doing is to say "This is what we should all recognize is how this country is supposed to work".
Today the idea of having a country without a written Constitution (with a capital C) may sound naive. "Without a Constitution", you may ask, "how do they differentiate between a legitimate act by an official, and an abuse of power? Do they just depose him whenever somebody does something they don't like? And if somebody wants to change the de facto constitution, does he just take some unprecedented action and hope nobody stops them?" Well yeah, kind of. If you want to see this in action, just follow UK politics, one of those 5 countries without an official Constitution - the others being Australia, New Zealand, Saudi Arabia and Israel.
In the UK, pretty much everything is just based on traditions and conventions. Officially, the entire country still belongs to the Queen, whose word is law, and all acts of parliament are mere "suggestions" until she signs them. Even votes of no confidence, passed by parliament, don't actually force the Prime Minister out of office, it's just a tradition that he steps down after parliament passed it. In fact, the idea of a Prime Minister, a "first servant of the crown", is also one that slowly emerged as one minister grew increasingly powerful during the 18th century. Today, the only accepted "legal" base of power in the UK is the House of Commons, which has the ability to pass Acts of Parliament which lay out more explicitely how things are supposed to happen, such as with the Fixed Term Parliament Act. So in a way you could say parliament is simaltaneously the UK's (real) Supreme Court. They do have a Supreme Court, but all it does is to reaffirm that parliament is supreme.
A Constitutional crisis ensues when a political actor takes an action, which one part of the ruling class claims was illegitimate and fights to undo it, while another part claims it was perfectly legal under the existing politcal framework. With a written Constitution, at least in theory, it should be clear enough: What the Constitution says is legitimate, everything else isn't. But without one, where is the line between just creating a new precedent that one part of the country may not like, and doing something that fundamentally goes against what was agreed upon before in the de facto Constitution?
Another famous state without a Constitution was the Roman Republic. As the republican era came to a close, Roman politicians started to over-use their veto power, the Senate gave unlawful term extensions to governors, and eventually emergency powers were abused when Julius Caesar became "dictator for life". Many of those things were brasantly unconstituional, but those over-uses of vetoes actually perfectly legal. But they were supposed to be reserved for threatening occasions, when the stability of the republic was under threat. Now bills were vetoed whenever a politcian just would have rather have not seen it pass. And as the political actors realized that using extraordinary powers on a whim was not punished or reversed, those powers started to be used more and more. And soon you had a very unstable political scene, where extreme actions were a regular routine: A situation that was supposed to be prevented through tradition. But if all you have is tradition, then once somebody dares to break it and that action is not undone, then the dam is broken as politicians realize they can get away with it, and they will inevitably keep using it to benefit themselves.
Maybe ancient Rome seems too distant for you, so let me give you a more recent example. What if the Speaker of the House of Commons in the UK, who determines what bills are voted on but is generally expected to be even-handed, decided that a motion can only be put before the House once in each session, and will deny it whenever it's put fortu again? Then unless the House replaces him with a new Speaker, that will become the new precedent, i.e. a part of the defacto constitution. The constitution was changed because one guy did something, and the powers did not intervene. That's actually exactly what happened recently; and if now somebody were to change it again, that would create backlash.
In a system that makes sense to me, a constitution can only be changed if a vast majority of the people, or at least of the political actors, agree to it. Basing a Constitution on traditions makes it more flexible to changes, but it also leaves the power to change how much power each of the actors have, to the very people who are already in power. If they change the conventions, that really means they do something and dare simply dare anyone to stop them. That stopping often only works through toppling them, like a vote of no confidence in a Prime Minister, or at least threatening to. And that binary choice of toppling or not toppling is fundamentally unstable.
In a Constitutional system, there is often a Supreme Court, that can declare any action or law unconstitutional, without necessarily toppling officials. That seems to be a much more stable and deliberate form of government, rather than the rule of the strong, who can create precedents in an informal system. But even that can stop working when the Justices on the Supreme Court don't call out unconstitutionalities, like how in the US the President nowadays has the power to spy on Americans and lead war without Congress, which cleary violates the Constitution. In the UK at least parliament can make anything legal or illegal, and they're democratically elected.
But then again, who guarantees that everyone follows the orders of the Supreme Court? We should never forget that ultimately an political system needs to be enforced. Even countries with an official Constitution also have a de facto Constitution, which isn't always the same. Without someone who has an interest in impartially determining which uses of power are legitimate, and which are abuses, power will always fall back into the hands of those who already have power to begin with. They use the power they already have to back up whatever de facto constitution benefits them. In that case, power begets power, and we shouldn't be surprised if one day somebody managed to get virtually all of it. Ultimately a Constitution is a wish list, and without a proper system to enforce it, it is not certain that the wishes will be met.
