In an ideal world, we would be able to navigate every bit of software from the keyboard and the increase in productivity and reduction in frustration would be glorious. In the real world, however and unfortunately, there are still quite a few applications (including the vast majority of the Web) that simply require a mouse^ to navigate.
Within those applications, any command that you execute frequently could potentially benefit from being bound to a mouse button. I used to have a Logitech MX1000, which had a bevy of buttons, and I bound all of them to browser shortcuts. My job involves spending a lot of time on internal websites and being able to do all the routine tasks of web browsing (open tab, close tab, open link in new tab, go forward, go back, etc.) with the same hand that already had to be on the mouse was very useful.
On the flip side, if you're on Windows and either spend most of your time with both hands firmly on the keyboard or want to add in the wondrous power of text-expansion, go with AutoHotKey.
^ mouse here means any pointing tool, including drawing tablets, trackpads, and joysticks