Firstly: 23..Bd6 24.Bxd6+ then black is in checkmate after a couple of moves.

 Then it seems like Karpov was worried about the danger of Ne6+ at some point if he goes 23..Nd7.

Anyway with Karpov's pieces being in such a knot with his queen being pretty much out of action after 23..Nd7 and with his king practically cornered with the threat of Rb2 at anytime not to mention Kasparov's other pieces prowing around. Karpov wasn't losing much by making such a swap he only went 2 of your famous points down infact and he gained a bit of space.

His back was against the wall when he went Qxc7 and he had to try something.

Other points knights may be considered to be the best pieces against a queen, with no knights behind her especially.

It probably seemed to be to be his best chance in a not too good situation PLUS the fact that he may well have been hoping that Garry hadn't counted on such an un orthadox swap.

PS I'm no grandmaster myself but when the chips are right down, then you've got to take a radical chance, especially if it's your only chance.

I see your point, thanks so much for the insight in this awesome game :)

Answered my questions :)

I remember this game. It is from Linares SuperGM 2001 (which should be indicated). 22.Bxd5 is correct, and is better than Nxd5. If 23...Nd7 24.Rh5 (24...Rg8 25.Bh6+ Rg7 26.Ne6+) Rxh5 25.Qxh5 Bf6 26.Bd6+ Ne7 27.Nd5

Cool game! Even at the end, where Kasparov trades his Rook for the Bishop to eliminate further Karpovs arsinal and attacking possibilities!!

Very interesting, I wouldn't have figured that there would be a way to lose a queen to make up for a last resort.  I'm starting to get what you mean.  This simply means that Kasparov's Bishop sacrifice on like move 22 was simply genius. 

simple, more pieces = more possible play

other lines would lead to positions where white is completely dominating and the queen was lost anyway if not a whole bunch of material trying to save it.

just a last resort to complicate things. Also, if knight blocks, white either doubles rooks or does Knight e6 check (not entirely sure on that one, it just looks like a threatening move after queen recapture) and if there's no holding attack on the king, at the very least after capturing the d7 knight used to block, white gains the material back. Though I'm guessing there was such an attack since he opted to take the knight, lol

Black wouldn't be able to defend the d7 knight with the other knight or rook because of the bishop on f4

I agree with the comment above, the position was absolutely hopeless for Karpov. He just wanted to complicate the position to force Kasparov into an unfamiliar ending. 

The game was allready lost by that move 22. That Q sack was just an attempt to do something to prevent a predictable ending.