My personal projects generally have a better test suite than what I do for work. First of all, I do it at my own pace, but more importantly - it makes the whole thing possible, as you said.

The nature of a personal project is such that I may not come back to it for months, sometimes even years. The first thing I always do is run the test suite. It's my lodestar. It helps me understand what state the project is in so I can confidently pick up where I had left it.

It is also my requirements documentation - as code.