The growth is absolutely the most beneficial, but the connections with others has been a big blessing too. I'm very grateful to find others working on side projects, as my family just doesn't get my need to do it.
Having others out there trying too is inspiring.
Aside from growth and learning. Seeing something that was just a spec of your imagination slowly becoming real is definitely rewarding from the get-go. But integrating other people's ideas and realizing that "you can actually do it" makes it even more gratifying.
As a User Experience Practitioner, my main motivation is to make user's life easier. That is the #1 goal behind any idea that I pursue. Something that I am passionate about.
While working on the side projects, for me the most rewarding things would be learning (that you might not get from anywhere else), and the satisfaction of putting in effort and seeing the people getting benefit from it.
Stash
NotesHub
Hummin'