I love my lens doors, and am really sad that my Tamron 28-75mm F2.8 doesn't have one - I'm considering cutting a slot into it, after getting spoiled by having it on my Pentax kit lens and 50-200mm. Yes, its for reaching in there and adjusting a polarizer or other rotating lens.
Not having it on the Tamron really stinks as my Hoya 67mm HMC circ. polarizer is "slippery" on top, I pretty much have to remove the hood to rotate it. That's probably my biggest gripe with that lens.