Why play b7-b5 immediately after a7-a6 in the Ruy Lopez?

Here's a diagram:

A good reason for immediate b5, I think, is so Black then is free to play d5 without pinning his Knight on c6.

Thanks, Anthony. That is helpful. Though I noticed that none of the comments gave an explanation similar to mine above. Is a pinned Knight not really a concern for most advanced players defending the Ruy Lopez?

I wouldn't call Bxc6 "wrong" either. It's just a different line. By backing off - Ba4 - White maintains pressure and threat of pin on the Black Knight. To ward this off with b5, Black awkwardly advances edge Pawns. This is the trade-off for White repeatedly moving his Bishop. Now, in the line Bxc6 I think Black is better to play bxc6 rather than dxc6. I think his d Pawn is better left to advance into the center.

Thank you for all your comments. I am beginning to understand it all now. But I left something out. For all you Ruy Lopez enthusiasts, if the opponent plays an early b7-b5, punish him or her with a2-a4. It will exploit a weakness in the queenside.

Quite interesting demonstrations, Anthony. Good point, James. There is no end to the subtlty and complexity of the game!

The only real "advantage" (there are more disadvantages!) of an early ...b5 is to follow with ...Na5 to win the two bishops, but at the cost of time and a queenside weakness in a soon to be opened center with d4. Most strong players prefer to wait a little longer, as it can only be inferior to play it right away if you don't follow with ...Na5 and even that is an inferior, though playable variation. Trying to play for ...d5 is too slow for black, as white will probably get there first with d4.