I have a Logitech MX Master which has a bunch of stupid side buttons that I really like. The thumb pad stops/starts whatever I have playing in MusicBee, depressing the thumb pad and moving the mouse left/right skips whatever I have playing back/forward ten seconds, depressing the thumb pad and moving my mouse up/down raises/lowers volume, the little scroll wheel brings up the alt+tab screen (I don't really use ti for that though because I normally have my left hand somewhere around the actual alt+tab keys), the forward/back buttons I set as the keys J and K, which you can use to navigate up and down posts in a bunch of websites (eg tumblr, twitter, the old reader), the middle button below the scroll wheel brings up Task View. Normally I don't bind the side keys in games because I find it more comfortable to use the keyboard for that sort of thing. I don't even like binding melee to mouse3 because hitting mouse3 over and over can feel a little uncomfortable... If I do bind the side buttons they're normally to something like a quest log and an inventory. To be honest, the forward/back side keys on the MX Master are a little finicky, so I don't think they'd be good for anything more heat-of-the-moment anyways???
Since my first mouse with side buttons (a ~2012 razer deathadder) i've never used them for forward/back in web pages. I rarely use the forward/back functionality on my browser anyway so I don't need it on my mouse