Why you add contactors in PLC controlled system while PLC has built-in contactors?

main reason is to isolate PLC

Plc has contactors with low load thats why we isolate the output by external contactors to protect the plcs contact and increase its lifetime. Also in case of running3 phase motors we will only need a single signal from one relay (contactor) to activate that external contactor for that motor.

to run a three phase (high rated) load we must need a3 phase cotactor which can be powered by PLC. single phase low power rated load may be directly powered by PLC.

PLC contact are of very low rating thatswhy external contactor are used, which could be of very high ratings. 

there is no contactors in PLC .