As the knight is already developed on c6, black can immediately create counterplay in the center: 4....Nc6 5.c4 Nf6 6.Nc3 Bb4.
Clearly, 7.Bd3 (the mainline after 4....a6 5.c4 Nf6 6.Nc3 Bb4) is not possible now, as black wins a piece by 7....Nxd4.
Instead, white has to defend the pawn on e4 in another way. Both after 7.f3 and 7.Qd3, black has the strong ...
Castling long as soon as possible
The Qf3 will most often move to g3 to pressure g7 and d6, and possibly help the break e4-e5.
Both central files are left for the rooks to apply maximum pressure (unlike after Qd2 or Qe2 developments)
For starters, the e6 pawn controls d5 and f5.
(1) This is great for defensive purposes because a knight landing on d5 or f5 can often ruin black's game completely in the Sicilian. A great example of why we don't play e5 and allow a knight to land on the d5 square would be the Sveshnikov variation. With our pawn on e6, white must find another square (and ...