SSR function con control panel
It's a wiring digram instead of a circuit diagram. These are not very easy to read sometimes. If you had a circuit diagram it might be clearer....

Anyway... it looks like the PID controllers are always on so the SSR will always be controlling the heater power circuit (12V, 1A for  example). This is not a good thing when the heaters are not needed.  

Instead of switching the PID controllers off the designer has switched the SSR power circuit on or off by using a relay. This is on the power side of the circuit.

The same effect can be achieved by switching the SSR control signal (The SSR LED circuit) or turning the PID Controllers off. 

Why switch the power side of the circuit? I can not tell.

A smaller relay (ie a signal relay (5V, 20mA for example) for the SSR control circuit) could have been used instead to save power and expense ...  

The likely answer is it is just as easy to do this and or it was a copy of an existing control panel ...