There are two theories.

1) The loud hubs help raise awareness of your presence and thus are safer than the ultra-quiet hubs, letting pedestrians, motorists and scooter riders know of your presence, and thus obviating the need for a bell or a simple shout of "HEY! ON YOUR RIGHT!"

2) It's a byproduct of designing ratchets with more engagement points for shorter engagement, such that when the ratchets are disengaged and spinning they just make more noise.