I feel that there is a world of difference between a side-project that is free, to one where you ask customers for their credit cards. Of all professional experiences, I have never learnt as much as I did taking a side-project idea from idea to MVP, then to beta users, then to paying customers, scaling server issues, and marketing strategies. It's not so much about the money, than the fact that you learn so much on so many dimensions.
Sincere thanks + shameless plug, the interview about Smooz was fun, a good self-reflection exercise, and a good source of traffic too (https://www.indiehackers.com/businesses/smooz)