Eight years ago I was really bored at work (they just asked for a couple of SQL queries a day) so I made a poor man's automated trading system for government bonds. The basic idea was to camp for new bonds and look for new opportunities as soon as they presented themselves. I had also done some rudimentary backtesting to prove some simple strategies for buying and selling. I made no money off of this, it was just for fun. During development I learned how to acquire, store and process financial data in a live streaming way as well as how banks handle things, what the fees and taxes are, etc. Also helped me learn web automation, making and deploying my own services to my own servers, etc.

A year ago I started working on hedge fund software and all this experience was a huge help. So if you are busy at work making tools, great. But the benefit of side project stuff is pretty situational.