Assuming you've trimmed the case before reloading, the flat-nose cutter of the trimmer is going to leave small burrs hanging off both sides of the trimmed surface. That is, on the inside and the outside of the case mouth.
If they are not knocked off, then they can cause you trouble. If one of the inside burrs breaks off during seating, then it can wedge between the bullet and the case neck, making a bulge that will keep the round from chambering. Burrs on the outside can do the same thing, or can cut your fingers.
As a person who regularly works around machine tools and freshly cut metal parts, let me advise you that you haven't "lived" until you've gotten one of those tiny pieces of metal lodged under your finger tip. Yow! Not fun.
So to me, the act of chamfering is not so you can get some kind of perfect angle and depth (like the trim length is), but to simply make sure all the burrs are gone. If you have one of those motorized case trimmers that spins the case, then slapping a piece of Scotch-Brite abrasive pad on their while the case is still spinning might do just as well as the cutter.
If you do lots of rifle cases, then you can really speed things up by putting your cutter(s) in a drill press and offering the cases to the spinning cutter by hand. Or you can ask Santa for an RCBS "Trim Mate" for New Years!
Hope this helps!