Pool drain

[Trsix]

Pumped out pool with a submersible and cleaned the pool to paint. My main drain on the bottom weeps a tiny bit of water.
My piping looks very simple by the pump. I've always wondered why there's even a drain on the bottom when there seems no way to use it. Here's a pic of my pipes. Water in and out. There is one additional 1" I think pipe sticking up capped off, no idea what that would be for?(my photo is too large I'll resize it and add later). Thanks

[Trsix]

Resized photo

[Teapot]

Is the main drain connected into the bottom of the skimmer? Two holes utilised in the bottom of the skimmer rather than one.

[Trsix]

After I did a search after reading your post. That's my situation.
2 holes so i stuck a tiny hose thru the floor drain n sucked water out ,done. Thx