With 5...a6 Black restrains White from playing 6.Bb5+.
It's a very flexible move with several functions. Most importantly it's a waiting move which enables black to decide on the course of action only after seeing what white has in mind, but it also helps tremendously in other ways by enabling a later b5 push and keeping knights off of b5 (which is critical since the queen oftens moves to c7). It does indeed have the benefit of preventing Bb5 but I don't know of a single line where Bb5 is a major threat. Even when black opts not to play a6 until much later, the move Bb5 is very rare. If black wanted to rule out Bb5+ he'd have to play 2..a6 (The O'Kelly variation).
Unless White plays 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Bb5+. 5.Bb5 is designed to exchange the Black N if Black plays 5...Nd7 or the Black LSB if Black plays 5...Bd7.
Bb5 is very rarely played in either of those circumstances. I will go even further and say it's not a good move.
In answer to your question of what to play against 1.d4 you can bet on the King's Indian Defense.
Too much maneuvering for me.
For example, if your Q is threatened with capture. There are 2 ways to defend it. The indirect way if it is available in the position is to threaten your opponent with mate on the move if he captures your Q. Very true. Would we then call the move that threatens mate hypermodern?
They're boring games. Plain and simple.
-Former reti & english player
We can't embed youtube videos anymore??
http://www.youtube.com/watch?v=glAhxGTgoKY
yup, even Kansha (staff member) is trying to figure this out