It's called a bottom bracket wrench, but in reality it does a few more things. Multi-tasking is good!

It's primary purpose, as the name implies, is to install and remove 16-notch bottom brackets. Shimano, SRAM, Campagnolo, FSA, Chris King, and others all use this pattern. It's machined to close tolerances so as to not mar the finish on your bottom bracket, and the offset handle is distanced just far enough so your knuckles clear the frame but not so far as to create unnecessary cam-out, causing the tool to slip.

The 1609/2BI-US also fits external Centerlock disc rotor lock rings for all those 15 & 20mm thru-axle wheels. And lastly, it includes a little wrench for the preload caps on Shimano Hollowtech-II cranksets. This handy tool has a strong magnet embedded in the backside to keep it attached to the main handle when not in use.