Well. I'm assuming it goes to a return line. There was, at one time, a chlorinator to the right of the check valve that I removed because it was leaking and I didn't use it. Although the clear plastic cover is hard to see through now, it is transparent enough for me to see bubbles bouncing around in there. I guess the Hartford Loop was installed for the chlorinator? I dunno.
I've considered cutting that loop out also. The short pipe between the two elbows at the top of the loop is some kind of flexible tubing that looks rotted and could become a problem soon. I couldn't figure out why it was there in the first place. It's also in the way when using the wrench to remove the filter cover, which is why I considered cutting it out.
It's a mess, I know. Kinda like me. Nothing but patch, patch, patch.