It is a sediment trap, and actually, you shouldn’t.

A long time ago, The natural gas that came to the house had a fair amount of sediment and moisture. At least I think it did, because plumbing codes required a method of removing said sediment before every gas appliance in the house. Want to see the code? Click: Here and read 408.4

The idea is simple; if the valve in a furnace closes on a bit of dust, the valve would stay open a little bit even after the furnace turns off. Raw gas would move around that dust and could ignite outside the furnace.

No good. So a sediment trap (or dirt tee) was invented. Gas from the gas company comes down a pipe, and the pipe takes a 90 degree turn into the appliance. Straight down from that is a section of pipe with a cap. Any moisture in the line misses the turn and drops into the lower pipe.

Thing is, our supply of gas in this country is incredibly clean. So the chances of a valve closing on a bit of dust is between zero and oh, never mind.

Still, for peace of mind, make sure that when a gas appliance is installed, a sediment trap is installed as well.