The downfall of standard contactors is the duty cycle wear and tear of the contactor itself. Electro-mechanical components have a finite duty life cycle compared with solid-state devices that have a much longer life due to little or no mechanical parts.
To keep a process at a certain temperature, some type of control is be needed to cycle the contactors on and off.
That control could come from a programmable logic controller (PLC), temperature single loop controller or something in between.
Standard silicon-controlled rectifiers (SCRs) will generally use a form of control known as zero cross, basically turning on and off at the zero cross of the sine wave.
The on-and-off function could be for one cycle or could sometimes use a zero cross time base function. Time base function has the SCRs fire for "x" time, and then off for "y" time, then repeat, as shown in the figure below.