I'm replacing my water filtration system. The old one used a dedicated drinking faucet. The new one connects directly to the main faucet. In removing the old drinking faucet, I found 3 lines: input, output and a third line that ran to a hole that was drilled into the drain pipe. This was held tight by a bracket that was clamped around the drain pipe.

My question is WHY and is there some reason I can't replace that section of PVC with one that's intact? Is there something that needs to drain there that I don't know about?