I find them vital for side projects - especially if they get set aside for a week/month/year, I sometimes will lose track of assumptions I’ve made or use cases for apis that tests tend to expose.
Sure you can encode all of that as comments, but unless you reread each file when you return from a break, you can’t always trace those thoughts and see where they lead. On the other hand if you “find all references” in your ide or change some implementation so that a test breaks, past-you can save the day with that extra information about what they intended at the time.
I really value tests on projects I work on alone because the work is usually intermittent. I don't have the time or energy to deal with regressions, and I may not remember everything about the system by the next time I work on it.
I also find manually testing to be tedious, so I'd rather spend that time writing code that does it for me.
Sure you can encode all of that as comments, but unless you reread each file when you return from a break, you can’t always trace those thoughts and see where they lead. On the other hand if you “find all references” in your ide or change some implementation so that a test breaks, past-you can save the day with that extra information about what they intended at the time.