Generally someone would change to a 5 bolt pattern on the rear end if they upgraded to disc brakes on the front. The 5 bolt disc brake kits are more economical and have greater availability.
Likewise if a rear end was changed for a better ratio it may require a change to the fronts to match. There is also a better selection of aftermarket wheels.
There is nothing wrong with the standard 6 lug wheels, drums, etc, and many owners retain them even on modified vehicles.
Fred
