OK, so I get that it's an accessory that attached to the bottom of the camera etc... but what does it actually do? I only ask because I've seen them selling for over $400 here and I'm confused as to their use now. Someone told me it was for "weighting" a camera etc, someone also told me it was useful for underwater shooting, buoyancy etc

But what are they actually for? Surely for $400 they'd have more of a purpose than just weight?

Sorry if it's a dumb question