why f3 is best move in this position?

Engine shows f3 should be played by white. what is the purpose of playing f3 now?

F3 is part of a whole idea of how to develop your pieces and attack.  Qd2, 0-0-0, g4, h4 and h5 is the start of a ferocious attack against black's castled king.  This is called the Yugoslav attack against the Dragon variation of the Sicilian defense and its very strong.  https://www.365chess.com/opening.php?m=13&n=1135&ms=e4.c5.Nf3.d6.d4.cxd4.Nxd4.g6.Nc3.Bg7.Be3.Nf6&ns=3.3.4.4.3.3.4.7427.7439.23890.19654.1135

To the extent you are asking about other natural looking moves like bc4 or be2, the question is a bit too broad for me to address in the air.  Also, candidly, a master might be able to give a better explanation as to those issues.  

thanks a lot man

Do not pay too much attention to the engine scores when there are several roughly equal moves (difference in score between different alternatives is small). It's true that 7. f3 is the most popular move also in human practice but white certainly has alternatives. 7. Be2 in particular if white wants to play a little bit more quiet game where both sides castle short.

If white wants to castle long then f3 is a usefull move to prevent Ng4 by black although I'm not sure if 7.Qd2 is totally out of question: say 7. Qd2 Ng4!? 8. Bb5+(8. Bg5) Kf8 9. Bg5.

You should never move your f pawn.

f3 is critical against the Dragon Sicilian. It keeps the knight out, prepared g4-h4 with a nice pawn storm on the kingside. 

stockfish

*stonkfish 

prepares g4, prevents Ng4

If g4?? Nxg4!! Wins a pawn.

obviously not g4 immediately...

Black is winning.

Black is dead lost here. I play the Dragon and the correct move is 9.g4...Be6!

I was joking, no sane player would play nxg4.