Do you mean tubes to deliver filament to the print head?

On our (Hyrel) 250-260C heads, we only use PTFE (Teflon) 2 mm ID tubing. Inside the hot head, we use PTFE tubing that mates with the brass nozzle (longer than standard M6s) where the heating and melting happens, and through the brass nozzle from there.

On our heads that go up to 450C, we bring the PTFE tubing to a polished steel delivery tube (higher friction than PTFE, but able to get hotter) which handles the heating and melting, then out through the brass nozzle.

So I guess the answer is that PTFE has lower friction (and is more flexible) than stainless steel. Depending on what's needed mechanically, sometimes one is better than the other.