I believe this is one of those things that are really hard to understand unless you have experienced it first hand. Although I always knew how important prioritization is, I understand it way better since working on my resource-constrained side project. You often fail to appreciate just how many obviously good things you can't do because there are even more important things to do.
Or, to quote Steve Jobs: "People think focus means saying yes to the thing you’ve got to focus on. But that’s not what it means at all. It means saying no to the hundred other good ideas that there are."
It's much more obvious with side projects because there is no one else to blame. You have a complete view of the picture.
I find personal projects very humbling. Even with no one stepping on my toes or setting unrealistic requirements, I still make dumb mistakes and write subpar code.
Or, to quote Steve Jobs: "People think focus means saying yes to the thing you’ve got to focus on. But that’s not what it means at all. It means saying no to the hundred other good ideas that there are."