Again, start with the visual pattern, this time from Black's point of view: White has two pieces—the two bishops—on the same rank. The one on a3 is loose. The arrangement suggests a possible fork by the rook. Of course Black’s own knight is in the way at d4, and would need to be vacated with a threat that forces White’s response; and the other White bishop—the one at f3—is not yet loose. But both problems can be addressed with an exchange. Black plays NxB; White replies KxN. In addition to bringing the king onto the same rank as the loose bishop, the exchange opened the d-file for Black’s rook. Now Rd3+ wins the piece.
A general point: When you have a chance to exchange minor pieces, as Black did here, as a matter of course you should play through the exchange in your mind’s eye to see how it leaves the board—what lines it opens, what becomes loose, etc.