Typically, a state convention will select persons to be delegates who represent their views and who favor their preferred candidates. That's the idea. It's not a process of bound or even formally pledged delegates, but it is a representative process where it is often known (and asked!) who delegates intend to vote for. Much like the presidential election itself in November, it is a method of representative democracy using a series of indirect elections.
Individuals who have not really been involved in the party before, and/or who prefer candidates that are disfavored by the state party, are usually not chosen as delegates. As a consolation prize, all of the people who were rejected as delegates will usually be given the ranked alternate spots. There is never any expectation that more than the top handful of alternates will actually be seated as delegates, because states are operating on the expectation that most if not all of their chosen delegates will be able to participate. That is what alternates are for -- a unity gesture for those who have been rejected as delegates, and a mechanism to replace a small number of delegates with their immediate runners-up if needed.
An in-person convention that has hundreds of delegates replaced by alternates far down the ranked list isn't respecting that representative process, it is overturning it to reverse the outcome. It is creating a body where states are "represented" by the very people they rejected, voting for candidates that the state parties decided they did not want.
By way of analogy, the gambit would be rather like if Bernie held out for the possibility that relatively more Biden delegates will be kept away from the DNC by the virus, and then tried to claim the nomination on that basis. Even though Democratic voters have, of course, elected more Biden delegates than Bernie delegates. Good luck getting people to accept that as a legitimate and truly representative result.
