The term "side-project" has a lot of possible interpretations:
A - Hobby project to play with some tech just because you find it interesting
B - Hobby project to play with some tech that might help your career in the future by learning it
C - Homework project to learn about the tech you currently use at work
D - For-profit project to hopefully earn some extra money on the side and keep it that way
E - For-profit project to hopefully earn enough money that will allow you to quit your day job and make it your main job
F - Portfolio-making projects to show at job-interviews as a junior developer
The OP is about case E apparently (maybe D).