There is one basic purpose for this. It is to make the velocity adjustable externally. Its not absolutely crucial, but its nice to have. When i first got the gun i didn't have it, and it pissed me off whenever i would have to take off the entire back block to adjust my velocity. I then bought this and am extrememly pleased with it. It's not crucial to have, but if you have the money, get it.
Buy it if you have the money. It shouldn't be first on your list of things to do though.
I agree that the Rex Sidewinder kit is a huge step up in convenience over the stock velocity adjuster, but it sounds like you were making things worse for yourself than they needed to be. You should only have needed to remove the cocking rod to adjust the velocity, not the whole back block.