It doesn't kick the bishop off that diagonal, and playing ...b5 later puts the bishop on another great diagonal and is supposedly bad for black (an explanation of this would be great too). So why is 3...a6 in the main line for black?
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1.e4 e5 2.Nf3 Nc6 3.Bb5 a6
Playing ...b5 later is not bad for black. Playing it immediately isn't so great, but Black almost always plays ...b5 at some point (unless White plays Bxc6 first!).
The main reason to play ...a6 early is that you can now play ...b5 any time you want to if you decide that you don't White to take the knight on c6. If you don't play ...a6, and then later decide that you want to kick the bishop away and start by playing ...a6, White can go ahead and play Bxc6. If you've already played ...a6, you can go straight to ...b5 and White doesn't get a chance to capture your knight in response.
Both diagonals a4-e8 and a2-g8 are good for white's bishop. Ruy-lopez strategy is to pressure on black's position by a4-e8 diagonal. Ruy-lopez claims this diagonal is more dangerous and annoying for black. Indirect pressuring on black's king and then on the knight on c6 is the idea.
The pawn on d7 is going to advance and after that the knight on c6 which is protector of e5 will be pinned by the bishop. It can reduce the speed of black's developing. After a6 white shall decide to exchange or backward the bishop. After backward, black has the ability to play b5 and escape from pinning.
Also, playing ruy-lopez without a6 is possible but history shows, it's better to play the main variant.
After ...a6, white has to make a choice to either play Bxc6 or Ba4. If Bxc6 then black has enough resources to protect his e5 pawn and maintain a stable position (if 4.Bxc6 dxc6 5.Nxe5?! Qd4 and black will regain his pawn with good position. And if white doesn't play 5.Nxe5 then black has the bishop pair and simple development; black aims to prove that Bxc6 was premature).
Thus white tries Ba4 instead to try to maintain some edge, he still threatens Bxc6 but waits until he protects his own e4 pawn first before doing so. Seeing all this, that's the reason why black plays ...a6 immediately. Black can play ...b5 when necessary to help protect his e5 pawn when time comes. Yes the bishop goes to another good diagonal, but black doesn't lose any tempo with ...a6 and ...b5 so it's even ground. If black plays ...a6 later instead, white can -under good circumstances- immediately play Bxc6 (trying to win the e5 pawn, which is what he was threatening to do anyway) but without losing a tempo by retreating to a4 first.
The idea of 3. .. a6 is to force white player to make a decision about own black Bishop already in this part of the game.
For example:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bc6
or
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1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4
Its two different variations with tons of theory in each of it.
If you want the evaluation of 3. ... a6 move - it is neutral one, not aggressive. In the same category is for example 3. ... d6
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1.e4 e5 2.Nf3 Nc6 3.Bb5 d6
Not aggresive not mean weak - it is a solid one.
In contrast to 3. ... a6 or 3. ... d6
moves like 3... f5:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 f5
3... Nf6:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6
3... Nd4:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 Nd4
forces the combination game.
To show the example of passive move in this position is 3... Nge7:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 Nge7
waiting for white action even creating some minimal troubles for own black bishop here.
To summarise it:
the purpose of 3. ... a6 in Ruy Lopez is to become more information about enemy black Bishop and as a result the whites plans for this game.
Here's a common variation:
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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3
The white Bishop is on a decent square. It attacks a center square (d5), the weak f7 pawn, and the square to which the Black King will likely retire (g8). However, that's all this Bishop can do. Its influence is limited to 6 squares and it has little mobility. Black has gained space on the Queen side. Bb7 is a nice move. Black can be thinking about playing c5 followed by Qb6, or Bb5.
In short, the position is very flexible for Black. Watch that f7 square, though!
The point of a7-a6 is to enter variations where white's light squared bishop will be kicked back to c2 and black will grab space in the center with c7-c5. See for instance this variation:
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1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O b5 6.Bb3 Be7 7.d3 O-O 8.c3 d6 9.Nbd2 Na5 10.Bc2 c5
It is a popular variation for black and black has plenty of ideas. A complicated battle lies ahead. The short term point of a7-a6 is to kick away the bishop from the a4-e8 diagonal. The long term point is to get these types of positions where both sides have plenty of ideas and chances to fight for the initiative.
The reason a6 is good is because it "puts the question" to the White Bishop. If White doesn't immediately play B(b5)x N(c6), it gives Black the later option of kicking White off the a4 to e8 diagonal by playing b5 later on.
Yes, White gets another good diagonal on a2...g8. But one of the diagonals is better (or less bad) for Black than the other, meaning that Black's option of playing b5 is a valuable one. Also, unless White chooses to stay on this diagonal instead of a2...g8, the bishop goes off the f1...a6 diagonal when he retreats.
In his prime, world champion J.R. Capablanca was the hardest person in the world to defeat (he once went eight years without a single tournament loss), even though he was not always victorious (earned many draws). The reason is that he made as many moves as possible to take away his opponent's options. This is one example.
One of the few people to beat Capablanca (in 1914, just before this time) was Emanuel Lasker, who took the knight immediately. Most people playing White don't want to do this immediately, because this limits their options. But a6 prevents White from doing this at a later, more favorable time.
