What is this purpose of this access pipe in my bathroom?
It's a cleanout port. Very helpful if you ever have a clog downstream in the pipe. The plumber can stick a snake (not an actual animal, a tool called a snake) down it to loosen the clog.

No maintenance required.

Water coming out of that port means you have a clog and the pipe is backing up. If the seal fails it can allow sewer gasses into your bathroom.