Why does RAB matter in Half Blood Prince?
Spoilers:
RAB stole the locket, a horcrux, as he/she revealed in their
note in the basin where the locket was found. Read The Deathly
Hallows to find out who RAB turns out to be. It is obviously
imperative the heroes (Harry, Ron, Hermione) discover who RAB
is/was to uncover what became of the real locket. It is imperative
since the locket is a horcrux and must be destroyed to have a hope
of defeating the nemesis in the books, Lord Voldemort.