Only you PB knows for sure, and he likely can't be found or won't remember. Not much telling really unless you dig it up. Are you having some sort of trouble that makes you ask? I wouldn't like mine to be that way, but just curious if there is trouble. It is pure speculation on my part, but it looks possible that the capped line could have been plumbed to the suction there at one time, but it's merely a guess.
I would suggest cutting out that union on the suction of the pump at some point, as it is likely to leak and become a suction side problem. I'm not saying it will, but the potential is high for that to be an issue down the line.
One common setup is to plumb the main drain to one hole in the skimmer, and have the other hole in the skimmer go to the pump. Your skimmer has two holes in the bottom, so that seems likely, though it is difficult to tell for sure without a fair bit of work. When you have this setup, there is an adapter for the skimmer you can get that lets you force more water to come from the main drain. That gives you more flexibility in how things work, but doesn't usually make all that much of a difference.
It'a right beside the return which begs the question, are all of your returns working?
If your pad is above the water level, which I suppose it probably is since there aren't any valves or check valves, you might be able to dig around the capped pipe a bit, cut the cap off and run a water hose down in it and find out where it goes.... ??
I have one return line that t's off at the closest two returns then heads to the third one. On the suction side I have one line, but also do not have a bottom drain since my pool is all 4.5' deep. We contemplated putting in a center drain for ease of vacuuming. The PB would have tied it into the other hole in the skimmer.