What does this pipe do?

We have an inground fiberglass saltwater pool with three suction lines; One skimmer and two round ones near the bottom on the side in the pool. I know on the left the valve controls water flow from the skimmer from one pipe. Assuming the second pipe is for the ones inside the pool. What is the pipe and the valve for on the right? Yes, I should know this but my husband is asking me because we’re having a problem with the pool and trying to solve it step-by-step. So any explanation of these three pipes and the valves is appreciated. I can’t find it in any of my documents. Thx!