There are already lots of great explanations here. I think your comparison to SP with shares is a pretty good one. Holding SP without using it will increase with inflation, but very slowly. Using it to upvote others earns much more, quicker, but you have to either do this manually or set up an autovote for authors you wish to support.
My view on delegation is that it's a way to support someone without any risk, because you can retrieve it easily at any point. In this way it can be used charitably with no feeling of loss to you if they turn out to be scamming or they misuse it. Then, of course, some people hire SP out for an income, so by delegating you can easily get it back if they stop paying. Again, no real loss to you.