Had to have one replaced on my CTS-V. Randomly the "Stability Control Engaged" would pop up on the Nav screen and one of the brakes would ever so slightly engage momentarily.
I don't know everything its used for but stability control is one.
In my Caddy it was buried on the steering column and required most of the dash to be removed, it was under warranty but they had it for 2 days so I'm guessing very labor intensive.
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3