When you've played enough chess and have developed some skill, you should quickly be able to recognize the defining characteristic of this position.
Simply put, Black has created a serious potential weakness on his dark squares. This has occured because all the pawns on the kingside (and the d-pawn) have been advanced to light squares. This creates weakness at e5, f6, g5, and h6.
When this happens, Black often depends on his dark squared bishop to play and defend on those squares. If he were to lose the dark squared bishop, the problem can get very bad.
In addition, White's dark squared bishop is in an inflexible position. Because white has placed his pawns on the dark squares of e3 and d4, White's bishop is immoble.
Your first thought in this situation should be to trade the bishops. That is at least a good move and improves White's position. The additional option in this exact position of playing Bg5 intending to play Bxg5 on the next move and play White's two knights against Blacks' knight and bishop in a closed position needs to be looked at.
Note: for similar reasons, Black achieved and advantage when the light-squared bishops were exchanged.
