italin vs sicilian

well, think about what is the main idea of Bc4 in italian? To attack f7 first, the weakest pawn.

If black doesn't commit with e5, then Bc4 is less effective, like this :

Bc4 is doing nothing now, and black will gain time on it playing for b5 (with a6 before), or preparing d5.

Of course attacking f7 is not the only thing about Bc4 in italian...

Thank you for your explanations. Have one more question. What happens if black after e4- c5, Nf3 plays d6 which is standard move for sicilian and then white plays Bc4? Is Nf3 necessary? Understand it must be a dumb question for someone who understand chess becouse i can feel that something is not right when this is on board but i would like to completely understand why.

*sorry Nc6 is it necessary? what if black goes for standard sicilian and white goes still for italian what is surely a mistake but novices play that often.

There is no such thing as the "Italian vs Sicilian".  Both sides determine the opening, not 1.  These early Bc4 lines for White against the Sicilian is called the Bowlder Attack, or in other words, the opening is the Sicilian Defense: Bowlder Attack.

It doesn't matter if Black plays 2...Nc6, 2...d6, or 2...e6, the move 3.Bc4 is weaker than after 1.e4 e5 2.Nf3 Nc6 because in the former, the e-pawn can still go to e6, whether d6 or Nc6 was played earlier, blocking the Bishop.  The whole point behind it, as already mentioned, is specifically the weakening of the a2-g8 diagonal because of 1...e5.

Take a look at the concept of weakening diagonals.  A diagonal is not weak if you can plant a line of pawns along the diagonal, plugging it up where every pawn is protected except the one at the base of the chain.

So, for example, if Black plays 1...d5 (regardless of White's first move), he bypassed the d6-square for the pawn, which means the 2 diagonals that d6 occupies are weakened for Black.  Those would be the a3-f8 diagonal and the h3-c8 diagonal.

If Black plays 1...f5, the h4-d8 and a1-h8 diagonals are weakened because a pawn can no longer be placed on f6.

When you look at it this way instead of opening names or move memorization, you will start to understand why the move Bc4 is a lot stronger after 1.e4 e5 than it is after 1.e4 c5.

just look at my stats as black against the italian-sicilian.

https://www.chess.com/explorer?moveList=e4+c5+Nf3+d6+Bc4&ply=5&gameSource=other&gameType=all&color=black&username=foofooes&origMoves=e4+c5+Nf3+d6+Bc4

Bruh