What is the purpose of 5. Nb5 in the Sicilian?

I see this Sicilian variation played all the time at the top level, and I don't understand why white plays 5. Nb5. It seems to serve no useful purpose to me, since black can so easily defend against the threat with 5... d6, which is a move that black wanted to play anyway.

What am I missing here?

I would be thrilled if white traded his awesome d5 knight for my terrible e7 bishop. I think the purpose of 11... Bg5 is that black acknowledges that white will never make this trade (good knight for bad bishop), and black figures that white is going to play pawn to h4 soon, which will make the bishop even worse than it currently is. So that's why black wants to get his bishop on the c1-h6 diagonal, so that it can be a moderately bad bishop, instead of a downright awful bishop.

Yeah, White won't, but 11Bg5 atleast brings the bishop in the game. Also if not Nb5 then what do you play? A Knight on b3 is terrible, you look at the theory, its the main line, your pieces are quite good but you need to do a lot of prep with both black and white to play this line otherwise you will get tricked and end up in a mess.

Why not capture (Nxc6) and then play a quick c4 and go for a Maroczy Bind type setup?

Bad pawn structure, you see after Nxc6 bxc6 he has such a wonderful pawn structure but on the otherside White has a terrible, and doing this in the opening makes no sense, the position after Nxc6 is just bad.

Black seems fine if not perhaps a touch better.

White is totally fine and the position is dynamic with chances for both sides. The knight will either reroute itself to the strong e3 square via c2 (or even c4 in some lines) or sacrifice itself on b5 (these lines are complex) The bad knight is not permanent. Yes, it justifies Black’s positional concessions (along with dynamic play) but White is still fine with decent chances to play for a win

White is going to play 6 c4 even if black thinks the d6 pawn is nothing to be concerned about. Thats just the way it is.

The main goal to provoke d6 is to block Bf8.

Just compare with Nf3 :

That makes sense. I do notice that black's dark-squared bishop in the Sicilian often ends up poorly placed when black can't get the d5 pawn break in.

Thanks for the explanation!

The Bishop is only terrible because of Nb5. On other moves, Black can develop it actively before playing d6.

Yeap... that bishop now pressures the centre indirectly, whereas it would have just been a passive bishop on e7.

Huh that is strange normally what I do is I take or move my knight back

Ah makes sense

i thought the point of nb5 is to gain time for c4 next. as white normally wants to play c4 against the sicilian whenever he can as this limits black's activity. yes, black can kick the knight with a6, but white doesn't really mind that. he usually has enough firepower to fight for the d4 square while keeping an eye on d5 too. 6 n1c3 seems to let black off the hook as it looks like a sveshnikov by transposition to me. the main way to spoil black's fun is 6 c4, which is why i think black players trying to unbalance the game as much as possible prefer the sveshnikov (nf6 first to induce nc3 thereby ruling out c4 for a while) over the kalashnikov.

If black wants a "spicy" game, there is still the f5 idea.

Remember this famous game :