I used it sometimes and when I read about c3 Sicilian I found out that it isn't a good response to c3, why ? It seems like you develope a piece and add pressure on d4. I thought about it as good move. Thanks in advance.
I'm assuming you mean 2 c3 Nc6. It's helpful to give specific moves or positions.
If you do mean 2.. Nc6, historically white has been very successful with the natural (and stockfish-recommended) 3 d4.
In the LiChess masters database, for example, it shows that after d4 white has won a whopping 48% of the time while losing only 18%.
From looking through the top games in that database, black's pressure on d4 is insufficient and the knight ends up being misplaced on c6, with white typically using it as a tactical target with a later Bb5 as black attempts to untangle his position.
With 1. e4 c5 2. c3 white is aiming to create a strong center with pawns on e4 and d4 (after capture cxd4 cxd4).
- ... Nc6 does not prevent white from playing d4 and the knight on c6 might later be kicked away by white playing d5. In any case after 1. e4 c5 2. c3 Nc6 3. d4 cxd4 4. cxd4 white occupies the center and is ready to develop his bishops. Black on the other hand is not fighting much for the center yet and still needs to make space for his bishops..
More principled replies to 2. c3 are 2.... Nf6 or d5 that take advantage of the fact that white cannot develop his knight to c3 anymore.
After 2... d5 3. exd5 Qxd5 black has a centralized queen which is not easy to attack (because c3 is played) and the game might develop into a typical isolated queen pawn game. If white choses to play 2...d5 3. e5, black gets a better version of the advance French, where the bishop on c8 is not blocked by a pawn on e6 yet.
After 2... Nf6 the game typically continues 3. e5 Nd5 where white gets a pawn chain on dark squares similar to the French, but black has an easy game attacking the chain with d6, Nc6.... and developing naturally. The same positions can also result from the Morra gambit.
Another option is to play 2.... e6 which might very well transfer into the French defense (but that's likely not something you are happy with if you are a Sicilian player).