It is required for zero crossing control. The heater is a pure resistive load and can be handled by a regular lamp dimmer. Your fan is an inductive load and requiers a different dimmer to run a motor like a celling fan speed controller. It is still a triac but has more stuff for inductive loads mainly a coil. Still much stuff on the web for this stuff google is your friend. AndyStill trying to understand why the diac is a requirement.