Here's a demo of how the side buttons work in macOS by default. Once you've gotten used to this feature, it's hard to get by without it. Compare to Windows, where those same buttons allow you to fluidly navigate back and forward in practically any window with a history. By default, they act as a sort of crippled middle click, consigned to opening new links out from under you when you least expect it.
If you use third-party mice with your Mac, you've surely noticed just how useless the side buttons are.
