It's a bit like an individual person taking the option of receiving a lottery payment in one large lump sum (and nothing else, ever), vs. receiving ongoing monthly/annual payments for some period going into the future. The new owner pays them one big lump sum and then the new owner gets all rights to license the content for things like car commercials on TV, streaming, reissues of box sets, radio, etc. Also for derivative works of all forms and basically whatever the new owner wants to do with it.