In Sicilian, I see white plays Bc4 after or before I castle. I saw a lot of players responding to this move by playing e6.
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1. Bc4 e6
In this case people usually respond with e6.
What is good about e6?
Doesn't e6 create a weakness on d6?
You have already mentioned one good thing about e6, it blocks the c4-bishop. It may not seem very important, but the white's attack on a2-g8 diagonal can be rather unpleasant later on, especially after white's moves f4 and e5.
Other important factor is the d5 square. Nd5 can be sometimes very unpleasant, so you want to avoid it. It potentially stops even Bd5 (for example after b5).
Black in these structure usually opts for a central break with d5 and e6 supports this plan. That's another reason for playing this move.
We shouldn't also forget the fact that e6 allows to develop the bishop on e7.
To the "weak" pawn on d6:
Here's the deal. You protected c5, d5, e5 and f5, you have avoided some nasty moves like Nd5 or Nf5, you have made some space for your bishop on f8... but there's a price for that. Your d-pawn has become a bit weak.
It's not such a problem, though. Bishop and queen is guarding that pawn and later you can even put a rook on the d-file. Also, as I said, d5 should be coming. That means the weak pawn won't be there forever.
I would say e6 has many more advantages than disadvantages... so I would go for it.
Conceptual Question and almost answered correctly by KMartin . This variation is a favourite of Bobby Fischer and is named as Sozin Variation of Sicilian . The main point in playing e6 is Black protects the f7 square . Let's assume Black plays g6 and then White plays f4 . Black develops the Bishop on g7 . White e5 , de5 fe5 and then Qf3 . So you see that f7 is vulnerable .
One more important function is e6 protects d5 square . Study more Sicilian Schevengien games and you would know it better .
In addition to given answers, there is a sacrifice on e6 that one should be aware of while playing e6.
Say after black plays Be7 and Nbd7, then Bxe6 fxe6, Nxe6 attacking Queen at d8 and g7 pawn and getting 3 pawns(e6, f7, g7) for a piece and also opening the king side.
The sacrifice is still on after black castles (say ... Be7, ... Nbd7, ... 0-0). Then after Bxe6 fxe6, Nxe6 the knight attacks Queen at d8 and Rook at f8, Ending up slightly better for white.
I have won few games with this sacrifice in local tournaments. It can come as a surprise if not aware. Better cover up for it.
White's light squared bishop "belongs" on f3. By putting it on c4, White prevents the bishop from finding that diagonal, but attacks the c4-f7 diagonal instead.
E6 defends against the latter threat, against the pawn on f7. I would play this either before or after playing bd7 and Nc6. Depending on how the play goes after be6, I would consider castling queen side to get my king away from the influence of that bishop.
