The goal of a side project can be whatever a person wants it to be. Make money. Learn something. Improve people's lives. Creative outlet. Your definition limits people's ambitions.
But when I look at side projects for myself, I look at who's making money already in a subject I care about and if I can do it better.
In the things I failed at in the past, I was chasing new things that I wasn't particularly interested in or didn't get the subject matter expertise required to succeed - one had an ad supported business model which just ended up causing me to try creating 2 businesses effectively at once instead of 1. Many people underestimate those things and I did that early on and failed.
As I've gotten older I mostly look at things where I can compete and just outperform versus trying to do something novel. A lot of people come to me w ideas and get discouraged when they find out that competition already exists for their idea.
Very hard to do something entirely new and succeed financially.