How a move like g4 can be affected by ..a6?

Foe example, why if black in the Sicilian has played e6 before a6, g4(Keres attack) is recomended, while if a6 is played before it prevents g4? Thanx for any advice 

P.S Whats the point of Keres attack?I assume it is not only a pawn storm on the kingside? I used it once against a friend and got crushed soon, then he told me i overexpanded and he took advantage of that! 

Could you post a diagram of the position you are talking about or perhaps the lines?

It might help getting the answers you seek.

In general I could say that g4 aides a kingside attack (yes the pawnstorm) especially when castled on opposite sides, aside from that it gains space...But really I would like to see a diagram 

Well, e6 blocks the bishops attack on g4, a6 doesn't.

But further down the line, a6 stops a bishop check at b5. If white can check on b5 it removes a black attacker of g4 which means the pawn doesn't hang.

That's what's coming up in my head anyway, I can't actually think of concrete positions that mean any of this matters...

In the Najdorf move order, 1 e4 c5 2 Nf3 d6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 a6 6 Be3 e6 7 g4 can in fact be played and scores well for white.

In the Scheveningen move order, the Keres Attack is a strong opening for white if you are a strong player to play it.

Playing g2-g4 rips both players' positions open.  Why play such a violent opening if you are at risk of losing to higher rated players for so many other reasons?

In the najdorf where e6 has not been played the bishop controls g4, so it would be a pawn sacrifice, unless you play this crazy variation:

After 12. Bc4 Qc7 13. Qd3 dxe3 seems strong, so white should play the Qf3 and castle variation I believe.

This is a lot complex