It actually does do something useful. It stops the relaying of a tx your node does not wish to relay. Just because a tx can still make it into the block does not make the relay data limit useless. It’s enforcing a policy set by the node runner and the version of code they run.

Bitcoin is a monetary network not a jpeg network. Just because Satoshi didn’t specifically say it’s for money only does not make all data equal. It’s a peer to peer e CASH system. Not a peer to peer insert whatever data you like system. I personally don’t care if people are relaying monkey jpegs but non-continuous CSAM is likely a real risk factor and a possible attack vector.