It is for personal or machine security.
If C1 fails C2 will stopp the heater and vice versa.
Since the spoles for the contactors isn't shown in the drawing I will make a guess that if C1 or C2 burns together it won't be possible to start the heater again until the contactor is "unstucked" read changed.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

In a heater circuit, there is typically one contactor that is used generally for on-off control, so subject to wear and welding. The 2nd contactor would be connected to a High Limit temperature safety relay and separate sensor that only drops out that contactor if there is a thermal runaway situation and because that contactor is not subjected to constant wear and tear, it is more reliable in an emergency.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

As stated by Jeff and additionally that over-temp safety relay is usually a latched relay that requires a human to push a button to engage it.

Keith Cress
kcress - http://www.flaminsystems.com