body terminal of PMOS in transmission gate
Why do we deliberately introduce body effect here?

We're not deliberately introducing the body diode - we're deliberately trying to connect it to a voltage so that the body diode is as much reverse biased as we can get it to be. This then allows the PMOSFET to block current when it has the drain voltage higher than its source voltage.
If we didn't do this the PMOSFET body diode would become forward biased as soon as the source voltage was lower than the drain voltage and, it would be a very good transmission gate / analogue switch. Look at the symbol for a standard 3 pin PMOSFET on the right: -

Can you see the parasitic body diode represented in the diagram? Can you see that if the drain voltage exceeds the source voltage by a few hundred millivolts, drain current would flow through the body diode and this particular configuration would be ineffective as a blocking device.