It is impossible to be sure, but most likely it is designed to power a pressure side cleaner, presumably one that does not require a booster pump. Do you have an extra fitting in the side of the pool that is not in use?
My first thought was a pressure side cleaner. But Linen has a good point too. The best way to tell is to open the valve and see if water comes out in the pool. If it does it's a pressure port.
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!