What hose is this?

I am changing my expansion tank. In the midst of doing so, I noticed that there is a piece that comes straight down out of the bottom of the expansion tank, and a hose L's off of that. This hose looks terrible and I feel I should replace it while I'm in there, but I don't know where this hose goes, what part number, etc... and it isn't on any of the diagrams that I've seen. Any idea what this hose goes to?