Pods and stuff, too. Pretty much anything waterproof that could get just plain nasty.
Then again, it was probably because the field paint was pretty grody stuff that needed soap to get the residue off, and I didn't have access to a hose and big bin.
As far as designed purposes, there's several.
Marketing. People will buy a set of backs/inserts or tips, but usually not a handful of near-identical barrels. Even just the ability to buy that way is a marketing point.
Manufacturing. It's easier to make 250 fronts for 10", 12", 14", and 16", and 1000 backs all of the same length, than it is 250 full barrels of each. Also, the control portion of a quality barrel must be finished straight and smooth, and with a tip slightly larger than the back it's not quite as important for some makers there.
Half as smooth as a great part is a quarter of the cost.
Also, often the backs only come in silver and black (bulk pricing on finishing) and the tips come in pretty much any color you want (cheaper for small parts; more parts in a batch).
I could think of a dozen other reasons, but that's the engineer in me, not the typist. :P