My slant on underwrapping is that, on a beach rod or spinning rod, in fact most lightweight rods it is only for looks, it serves no purpose at all, and it can change the action on some rods. An old school of thought was that it protected the blank from being cut by the guide feet, if the guide feet are filed correctly (easy job) then its not necessary.
Some Big Game rods even have triple wraps, whole different ball game, the angler can't move very far in the cockpit of the boat, he has a massive fish on, twisting and turning at the rod, it can test the skill of any rod made by the best of rod builders, so strength on the wraps is needed.
btw....Triple wraps are made up of one wrap around the blank and two around the guides.
Personally, I think the days of fancy butt wraps and glitzy rods is over, IMHO Rods are a tool of the angler and what ever suits the guy who buys it, fine! I wont knock him for it, but for me, black on black is good and the beauty (if that's the right word) is in the overall look and quality of the build
One thing I will say in favor of underwraps, if you do break a guide on a special rod and want it repared to look as new, then its easier to wrap the new guide in the same place. sometimes, trying to repare a guide and get it in the right place and not mark that gap between the feet can be a pain, you need to take all the old high-buid off, if you dont, then the fecking new thread wont sit right
Tom.