Deburring is often necessary AFTER case-trimming. Many cartridges stretch on firing, and thereafter will not chamber in the same rifle or fireram. For that reason, these cartridges will need to be ful-length resized, my Swiss 7.5x55 cartridges is one such case. Many tight-chambered rifles will not take even the full-length resized cased, and therefore need to be trimmed to the correct length. This process, using an axial cutter that removed material from the neck in an even pattern, produces a ridge internally and externally that has to be removed using the suitable tool - called an inside and outside neck chamfering tool.
Remember, too, that when a case stretches, that stretch has to come from someplace, and that is the thicker material of the headstamp end of the case. Eventually, you will see a lighter-coloured ring develop there, indicative of an untoward amount of stretching having taken place - that case MUST be then be scrapped, as you are looking at incipient case head separation, something that you definitely would NOT like to experience under any circumstances.
Hope that helps.
tac