Why were plc's developed?

"PLC" is the replacement of sequential ckts(and,or gates).relay logics.before the development of plc,s relay logics uses approx 1000,s of wires and 100,s of connections.and the main problem with relay logics is that these wires wered replaced and repaired annualy ,which is a very difficult task and time consumming.so plc is developed to solve the problem.(by faheem khan)comsats abbottabad