As a TT rear end without paying 800 for leader gears. Its also handy if you cant find a complete na rear end. The q45 diff allows you to mix the output shafts of the TT and knuckles with the NA subframe and gear ratio and I think keep the TT drive shaft. Silva guys like to rob NA z's for wheel bearings or to upgrade to 5 lugs so the option for a full rear isnt alwayz there.
"torque gets you off the line...& hp gets you over the line." - TTKeeper