Hi: Cooking prawns toughens them up so that they stand up better on the hook. There is very little difference in the small of cooked vs uncooked prwns. Cooking gives them that excellent orange pink colouration.
So why would a person go to the trouble of getting unkooked prawns.
Cooked prawns have another distinct advantage. You really only need a small piece of prawn on the hook behind a spinner or fly ,the rest of the cooked prawn makes an excellent snack while on the river.