Had my MBP for a few days and so far besides adjusting volume/brightness in one swipe, Touch ID, sleep, and default app-specific functions, I've been using it with Better Touch Tool for
- app switching (nice if you have a lot of apps open since you can tap the icon rather than scroll to it)
- seeing estimated remaining time on battery
- restarting wi-fi in one tap
- bluetooth on/off
- emptying trash
- opening Windows 10 in Parallels
- one-tap regional screenshots
- starting certain apps
- opening websites in Safari from anywhere
- music stuff
- custom macros to automate things in unsupported apps
I've seen more clever stuff from others that automate things using Automator, AppleScript, IFTTT, etc. There's no shortage of uses. It just depends on your awareness of its capabilities and your needs. I didn't know how much I could do until I saw other people's customization.