1. Your users leverage unintended behaviours, which obstructs your ability to move forward with meeting needs with intended behaviours in your intended target market (unfortunate but very real)

2. You know if you add or improve existing features, you can access more users, or an prospective user worth more than the existing ones

3. It's a passion project and you want it to function/look/whatever a certain way

I've encountered all of these, and each time it becomes a pretty big pain to make progress.