you find the plane you hired is not there, or is half an hour away, or it's got 0.8 hours left before the next 50, or something doesnt work, and so on
Half an hour away will happen from time to time in an environment where people are taught that rushing and hurrying introduces unnecessary risk. Your own schedule might be safer it it had half an hour slack in it so that the delay didn't matter.
The others are reasons to choose to rent from someone else if they happen more than very very occasionally (and the 0.8 hours shouldn't happen at all, they should have warned you about that when you made the booking).