What is the need for using O:2/0 at the bottom of start
It’s a latch used to hold the output on once the momentary start button is pushed.
It is used for holding the logic ON state.
When you press START I:1/0 the Output O:2/0 gets ON. But when you release the I:1/0 the Output 0:2/0 will also get OFF.
When you put holding condition(O:2/0) parallel to I:1/0 even if it turns OFF the output will remain ON state. Till you Turn ON STOP I:1/2 input