What are the operations and functionality advantages and limitations of plc?

1) As PLC's have solid state switches, they can provide unlimited switching cycles so they can be used to create PWM signals.  

2) PLC's are re-programable

3) A very complex logic can be created without the need of complex wiring.

4) Multiple devices are embedded in a single PLC like Timers, Memory Shells etc. and also multiple sensors can be attached like temperature, humidity, motion, speed etc.

5) the function of switches connected to PLC's can be changed as they are re-programmable, also the connection between a switch and a load can be changed in software. If all the lights of a house and all the switches were PLC controlled (E.I.B. systems) then you can decide witch switch shall control witch light without the need of re-wiring.