Removing the bolt makes it possible to clean the bore from the breech (always preferred) instead of cleaning from the muzzle. This is for detailed stripping and cleaning. This is done maybe a couple of times a year and always after firing black powder loads.
Yeah, I know. Being lazy, I almost always clean from the muzzle, and just brush and wipe out the breech area. This is the routine cleaning after shooting, and seems to be enough to keep the rifle accurate enough for hunting.
John