Bb5 is basically an Accelerated/Delayed Grand Prix. Whereas in the Grand Prix proper, White usually plays 3 f4 g6 4 Nf3 Bg7 and now 5 Bb5...
Burgess Defines the Grand Prix Attack as White 3. f4 with Nf3 and Bc4 | Bc5 to follow. Is 3...g6 necessary for Black? Wouldn't Najdorf style allow 3...Nf6? So we have:
Bb5 can be a good idea but this is not the main reason why d6 gives an improved GPA for white, at least compared when black doesnt play d6.
Some ideas :
U can notice in this final position that black cannot play d5...
This position is plyable for black but can be scary. Moreover, white has some alternatives before but i can't explain everything.
So u can now understand the point of Bb5 on 2nd move : to avoid the first diagram!
Unless white wants to go for the Bb5 variation in my first example, 2.Bb5 leads to other positions.
If black doesnt play Nd4, then Bxc6 doubling pawns, and white will continue in GPA mode with f4. Thus we see a mix between Rossolimo (with Nf3-Bb5), and GPA (with Bb5 and f4). This could be an improved Rossolimo if black enables Bxc6. So Nd4, which seems to gain a tempo on Bb5, but this ia the beginning, and u can see my link, where Bryan Smith explains why Nd4 doesnt really win a tempo for black...
Guys I found the best way to combat the Siclian Defense...
I stopped playing e4!
In #30 Blinsk shows that backing up B is a bad idea, which means it has to take N, or retreat. Doesn't taking mean white loses his best B for black's worst N? And isn't that extra pawn in the middle useful for Black to build central control? Black suffers a doubled pawn, but has these other pluses...
If white B retreats down his original diagonal hasn't he just wasted a move?
In #30 Blinsk shows that backing up B is a bad idea, which means it has to take N, or retreat. Doesn't taking mean white loses his best B for black's worst N? And isn't that extra pawn in the middle useful for Black to build central control? Black suffers a doubled pawn, but has these other pluses...
If white B retreats down his original diagonal hasn't he just wasted a move?
What are your criteria on a "best" and "worst" piece?
To my poor knowledge, "good" and "bad" pieces are strictly defined by the pawn structure, and here the pawn structure is as flexible as it gets.
Shallowly, one may assume that the Bf1 is a problem piece because it prevents white from castling, so white is willing moving it out fast, and possibly exchanging it for a knight, to accelerate his development.
In the Chigorin and Breyer Ruy, it frequently happens white's dark-squared bishop being the piece which orchestrates white's kingside attack. Well, that bishop in many lines does not move at all from c1 until move 20, or even later.
Good points pfren, your "poor knowledge" is less poor than mine. I'm just repeating stuff I've read in questionable beginners books 
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These books say white's white square bishop is generally considered strongest, and having two bishops rather than Knight & Bishop is generally a good thing. But your comment sounds right to me. It surely all depends on prawn structure (and development?)
Also, the Ruy Lopez exchange variation is considered a good opening for White, so my talk of "best" and "worst" doesn't really work. I withdraw my comment and bow to your greater knowledge.
in chapter 5 (A1) of "Fighting the Anti-Sicilians" by Richard Palliser he looks at this variation. You can get the book at everymanchess and read it with your free chessbase reader 2017.
1.e4 c5 2Nc3 Nc6 3Bb5 Nd4 4Nf3 is suprisingly effective, at least to me, Think called Tiviakov Grand prix. My opponent in this game plays it quiet a lot https://www.chess.com/analysis/game/daily/221712150 Responded with line recommended in Quality chess Anti-Sicilian book, until he played rare move with 10b3.
Um I think this is the Sicilian closed variation traditional.
Burgess says Closed Sicilian is defined by 3 g3:
He mentions several third moves for White as very damaging, but not the OP move. If you play Najdorf as Black, he says 2...d6 is the only move against all White's third moves, so I show that here instead of 2...Nc6.