llCollisionSound is a function that allows an object (or a linkset) to have it's physics collision sound (or sound volume) modified. (the "thump thump thump" you get when you climb a flight of stairs).

Currently, llCollisionSound only works on a per-prim, or per-linkset basis. Once prims are unlinked, only the former parent prim will have it's llCollisionSound changed, with the other prims in the linkset reverting to the sounds associated with their material type (wood, rubber, glass, metal, flesh, stone, plastic).

It can be desirable as a creator, to set the CollisionSound for all the prims, and not have to worry if they get unlinked or relinked in different orders during building.

being able to run a command like llSetLinkprimitiveParamsFast(LINK_SET, [PRIM_COLLISION_SOUND, "", 0.0]); would allow the creator to set the sounds and volumes for ALL the prims in the linkset, in one fell swoop, ensuring that they wouldn't need to tediously go around to every part of a park or region build that might get unlinked at a future date.