Wouldn't surprise me if it was done to facilitate easy loading of conicals or other non-spherical ammo. Lead balls may cut in easy but a square base round is much harder to accurately seat without the chamfer.
It also reduces the chance of damage if you're careless and don't properly align the cylinder before ramming. Yes; proper technique will avoid this, but how often have you seen the wonderful 'tube vids of people who shouldn't be allowed near firearms.