The steam in the smoke stream condenses on the relatively cool metal surface, and the smoke becomes drier, cooler, and presumably therefore, tastier. The downside is that the condensate accumulates in the shank and bowl of the pipe, which results in gurgle, and a wet, unsmokeable tobacco heel. It's a problem that you can't solve with a quick dip of a pipe cleaner because most stingers block its passage.
Savinelli's balsa insert system, and Brigham's rock maple tube inserts, to name but a couple, are fairly effective at capturing much of the moisture from tobacco smoke, without aggressively filtering any of the compounds that contribute to the taste of the tobacco.
It would seem that research in this area has been going on for a very long time. Depicted here, one of my favorites, is
marketing pioneer Daniel Boone Savinelli with an early focus group.