I think the other part worth mentioning is that you tend to get a drier smoke- the smoke travels at fairly high speed through the draft hole, hits the chamber, slows down and condenses out a lot of the moisture- I've smoked my RC Snappin' Turtles 5 or 6 bowls in a row at pipe shows and never touched them with a cleaner...
The Pete system pipes use the trap to do basically the same thing...There can be a huge variation in the secondary chamber size and they all seem to do a pretty good job....