Even if you have an income stream that makes up to 90% of the money. When working for yourself, you'll need something different for that other 10%. This not only provides an outlet for creativity but diversifies your income in the event something happens with that primary revenue source. Again, this may not work for everyone but for ME and MY EXPERIENCE, being able to branch out into other ways of making money besides freelance writing made my time as a solopreneur a lot more bearable.
I surprisingly get a lot of people asking to detail their car. I never intend to make a business of it, but I love doing it to my cars and people ask me to do it to theirs. All it takes is a cheap orbital buffer (mines a used craftsman) and a shop vac. I normally get easily $100 for a basic wash/wax/vac, or $200 to remove scratches and polish then wax the car.