Why do early Bxf7 sacrifice?

Someone said early Bxf7 is a good trick and I do not understand at all. Why lose bishop from the beginning? Or early Bxf7 is mistake people doing?

It all depends on the position on the board and what white is getting out of it. At a minimum it will be 1 pawn plus black can no longer castle in exchange for the bishop. Sometimes it is fully justified because there are other threats coming, sometimes it is unjustified because black has time to organise his position. For example:

1.e4 e5 2.Bc4 Nc6 3.Bxf7+ Kxf7 4.Qh5+ g6 white has not got enough.

1.e4 e5 2.Bc4 Nc6 3.Nf3 d6 4.0-0 Na5 5.Bxf7+ Kxf7 6.Qh5+ g6 7.Qxa5 white has won his piece back plus an extra pawn in addition to black losing castling ability.

Sometimes it can be done with another piece:

1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nxf7 Kxf7 the Cochrane gambit. White has 2 pawns here for the piece plus the misplaced black king. In this instance black should be at least equal if not better and so it is probably an unsound gambit, but it is not easy for black to play and only a slight mistake can lead to white having the better chances. 

There are some ideas with the italian game:

But I think its not a very good strategy... The thing is that you will put the black king in a bad position, but you will be down in material.

So if you play like that, you have to play with a lot of energy and attack!

Low rated players do it in bullet to because they have memorised an attack that whilst weak will be strong if you don't know how to defend it.

In bullet it bores me to death.''''''''''..only for patzers.

Chess isn't about 'good tricks' in general.  So whether it is or isn't a good trick doesn't make it a sound move to play in a proper game of chess.

Thank you for showing it!! I was wondering it!