Without more details on the application, multiple microcontrollers can be used in a modular approach. One microcontroller per task; it makes for simplified programming = lower costs.

There are many other advantages: you can replace one defective module, no need for expensive high-end multi-feature microcontroller, easier to add future modules, etc.

I always design in a modular fashion. It's so much easier to program, debug and repair.

Robert

(I have no experience with PLC)