Why take the knight here?

Often I see this early in games. The Bishop pins knight. Then whether I move Bd7 or a6, the bishop usually takes the knight. The Bishop is then taken. 

Can anyone explain the theory behind this trade?

Thank you,

Callum

It's an advanced tactic that people copy, but they don't know why. The answer is that in the aftermath, there is just a pawn and a rook preventing the pawn on A2 from being a pass pawn.  Either that rook stays there to prevent that from happening or the rook is moved just leaving the pawn there. 

Also some people don't like the opponent having knights because knights can be more dangerous than bishops against some people.

There might not be a good reason. Firstly e3 looks a little timid, why not e4? The knights control central squares so capturing c6 improves whites control of d4 and e5, maybe white has plans to exploit this somehow but he needs to kick his own horses into the game first. In fact white risks falling behind in development. White might also undervalue his Light Square Bishop, the position suggests d4 which puts his pawn structure on dark squares meaning he may be about to trade off his good bishop. As it stands I prefer black. After the trade he gets a half open file for his rook and an extra pawn towards the centre. Blacks LSB gets plenty of scope and might slow down kingside castling if it comes to a6. Suggest black develops rapidly and attempts to open up the centre.

Thanks for all your answers. Being a new players thus has confused me for a while.

Typically to avoid doubling my pawns I move my light Bishop behind the rook to remove the pin and use it to take the What would you do?

Disclaimer before I answer the question; I am not a master or even expert player but I do know and have read books on chess theory and opening fundamentals. Personally, there, I would not worry about the doubled pawns and move my pawn to e5, taking the center. If the light pieces take the knight, the doubled pawns are not so much of a problem because honestly, he/she hasn't developed any pieces ( and in essence have traded away the only developed piece on the board, not including the queen of course) and that pawn supports or strengthens the pawn on d5. Plus you get an open file for your rook. 

     In the diagrammed position white likely intends to play d4. Removing the knight on c6 hampers black's counterthrust ....e5. The exchange also opens the possibility of eventually putting a white knight on c5, perhaps making it difficult for black to place his pieces advantageously in the center. Also, white may play c4 and exchange on d5, leaving a backward pawn on the semi-open c file as a weakness that can be pressured.

In the diagrammed position Black's best is probably 1...e5! grabbing control of the center and laughing at potential double pawns! But 1...Bd7 isn't bad either. After 1...Bd7 I see no point whatsoever in 2.Bxc3