Why is ...Rb8, and not a6 used to support a b5 advanced in the closed sicilian?

Thread title explains it all don't you think?

A little more detail (specific variation?) would be handy, but most likely it's because White typically fianchettos his king bishop in the Closed Sicilian, and Rb8 gets the rook off the firing line of that bishop.

agree with spiffe. also, often times the b-file opens up after a b4 push and exchange on c3.

Nah, I'm not too interested in figuring out a specific line (I don't even play the closed sicilian) I was more interested in finding out some of the ideas behind the move.  I guessed the B file could open, but I never thought about getting off the bishops diagonal!  Thanks!

But the biggest reason is because white can probably play a3 preventing ...b4 when a5 needs to be played so it saves time, in addition to the other reasons.