I think it's an RC snubber, which is designed to protect the triac by shunting the inductive kick that occurs when you turn off an inductive load. It's the same idea as using a reverse biased diode on a relay to protect the transistor that turns it on and off in a DC circuit, but the RC snubber works with AC.
The thing is a heater usually has very little inductance, so I don't think it's very necessary?