I'm not a Bridge player, but I have been reading about the game. This part of the rule surprises me. After the contract is closed, Dummy lays down his/her hand and thereafter doesn't take part in the game anymore.
From a game design perspective this looks silly. It excludes one of the players from the game. Dummy essentially becomes a spectator, which is certainly less fun. If it can't be avoided (e.g. making an incorrect accusation in Cluedo) then yeah, but Bridge offers an obvious way to keep Dummy in the game - just don't have Dummy drop their hand after bidding finishes and play normally.
What is the advantage of having Dummy?