Yea, its used to reduce flow of water down since when its new its not really making a lot of bubbles, and can possibly make it leak at the adjustment knob. The thing is it was intended for use with the "old" style skimmer body which were notorious for leaking around the red adjustment knob. On the new style skimmer body with a seal on that red adjustment knob leaks are not gonna happen, and that piece with the valve built into it is not really needed, but its still included. One thing I would make usre is that yu have the NEW styule skimmer body. Its not impossible to order a CSS 65 today and still wind up having a old style shipped as stock lays in store houses and vendors for long periods of time. The CSS skimmers at one lfs here in town are all OLD style since they do not sell many of them at $210 for a CSS65 ;-)
The new style skimmer hsa a series of raised bumps on the top of the red adjustment knob. The old style simple has a falt topped adjustment knob voild of any bumps. If yu habve the old style call coralife and tell em you have a leaky skimmer and want a replacement skimmer body,. they will send one free of charge.