Why do they take the knight?

If its a bishop for knight exchange, how is that losing a piece?

Also, the exchange can depend on the position, and the player.

Low rated players do it because typically they capture whenever, and as soon as, a capture is possible.

High rated players do it because in that specific position the knight is worth more. It may be worth more because of something good it does (typically one of the central squares it influences), or it may be worth more because there are no other prospects for the bishop.

There are also other factors like pawn structure and time. Sometimes it's better to trade just for the sake of not losing time.

Although yes, often retreating the bishop (either to maintain the pin, or not) is a good move. When you retreat without maintaining the pin, you're usually arguing that the pawn move represents a structural defect.

Beginners confuse "trading pieces" with "losing a piece". 

If you lose a piece but the opponent also loses a piece of equivalent value - nothing was lost. The balance is kept. 

Knight for bishop is an equal trade, at your level. 

Make sure you don't just give a bishop for nothing, though.

If you can take a knight for nothing, or for a pawn or two - go ahead and do it - unless this involves bigger problems. 

Trade material on equal values or for your own benefit. Don't give anything away. 

One knight is worth three pawns. One bishop has the same value. 

A rook is worth five pawns. A queen - nine pawns. 

Even the loss of a single pawn for nothing can spell trouble. But pawn for pawn is okay. 

Before any clash between the pieces - stop and try to count. How many "units" am I going to take? How many am I going to give? Is it worthwhile?

If it is - go ahead and do it. If it isn't - do something else.

(as an aside - we say that the bishop and the knight are worth a bit more than three, and that a rook may be worth a bit less than five, so that a rook equals "a knight and a half" or "a bishop and a half". 

In my opinion, this doesn't apply to beginners. 

A beginner can sometimes use the rook or the queen rather well - but the knight and the bishop are less well understood. Therefore, the values above are probably an excellent guide for the beginner player, as they slightly overvalue the rook over the lesser pieces).

i do it because i dont want to lose a tempo retreating my bishop and i see no need for the bishop anymore

I do it for five reasons: 1) I generally have a stronger end game than most opponents at my level...the games I lose tend to be already lost in openings, so with fewer pieces on the board my chances go up 2) if I think the piece that they have to retake with (the queen likely in your example) will end up at a worse position than it was before 3) to maintain a tempo because it is my turn after the re-take, particularly if I see a good follow up move. 4) I know that in the current position his knight was better than my bishop or vs..... 5) I know the person I am playing is good with their knights

Backing away is sometimes bad, because it either invites ...g5/..b5 or g4/b4 with tempo (in some positions weakening of the white/black squares caused by it is justified), or makes your bishop "bad", using ChessNetwork's terminology.

That said, players that always take the pinned piece without thinking, probably, just lack a bit in positional understanding. 1. e4 d6 2. d4 Nf6 3. Nf3 Bg4 4. Nc3 Bxf3?!, for example, is a pretty lousy decision, in my opinion.

The Bishop is not necessarily a better piece to have than a Knight.  Additionally, when a pinned Knight at c3, f3, c6, or f6 is exchanged for a Bishop, it often means the b2, g2, b7, or g7 pawn takes the Bishop, leaving a bad Pawn Structure for the side with the Knight.  Additionally, backing away the Bishop can sometimes get you in trouble when your opponent can or threatens to trap that Bishop, like this game I won where if 8...Bg6 9 f5! wins the Bishop for a Pawn:

No saving black in that game!

The knight on the f3 or f6 squares can become a valuable defender of the castled king. Sometimes it's good to exchange it for a bishop that doesn't have good scope to weaken the fortress around the king. I often debate whether to take the knight, and often when I don't it gives me problems.

<Yorygregovich> you are right - this is a very valid principle of play!