I have a mouse with 6 buttons on the side and it's great for gaming. When I used to play fortnite I had all the building mapped there so I could do all my building with my right thumb and it wouldnt distract my left hand from movement controls
Whenever I'm working on a big photoshop file and I find myself using a tool very often, I bind the side buttons to that tool. At minimum I always keep the redo, undo buttons binded to the mouse.
Whenever I'm browsing the web I have forwards and backwards binded so I don't have to move my mouse across the screen to go back a page.
It's all trivial stuff but it makes my workflow slightly more fluid. I initially bought the mouse for gaming, but it has been super useful for me outside of gaming