A side project is interesting, shows initiative, and if it comes down to you and another otherwise equal candidate - it may well be the deciding factor in a hiring decision.

On top of that, hiring aside: the skills and expertise I’ve gained from a side project have been invaluable, and even for this reason alone I’d say it has been well worth it.

Most of the things I've had to do at work are very mundane things. Many boil down to shoveling JSONs around from one system to another or dealing with another former coder's bullshit. The actually interesting, hard tech knowledge I have comes mostly from spare time explorations.

I was asked at an interview about CANbus once, and that's not something I would have known from work, but I aced the question because I did quite a bit of side project hacking at reading a car's sensors.