Canyons involve a lot of sliding, scraping and general thrashing - which ends up with a lot of scraping and thrashing of the gear and clothing upon which that sliding is done. The harness, the shorts, the wetsuit - they all get destroyed on a regular basis.
The Scuttle Butt intervenes between these valuable pieces of gear and the abrasive rock. It works with many leg-loops-and-waistbelt-style adjustable leg loop harnesses that have leg loops that fully open. (The Scuttle Butt does not work with BD Alpine Bod, Bod, and 2012 Momentum AL harnesses.) Often the back holdup straps of harnesses will need to be cut or modified to use the Scuttle Butt, so when putting the Scuttle Butt on harnesses, the harness will no longer work on its own.
The Scuttle Butt extends the life of valuable pieces of equipment such as your harness, your wetsuit and your shorts. It also provides a bit of cover and padding for the butt, allowing you to slide, scrape and shimmy with impunity. To me, this latter is the greatest benefit.