several purposes is suppose, if your starting a new carreer in producing then it's a good idea to get a well known producer to do a remix for your b-side, this is for the simple reason of getting people to listen to it, i know some people will only listen to records that are made by people they have heard of and not by up and coming artists.
Also as a punter its good to hear different producers doing their version of a tune, i know personally i like some remixes where i dont like the original tune.
you can tell its sunday, my mind has gone blank now somebody else can have a go