Keeping the bishop is important because it protects the center. Eventually, Black can develop the rook on a6.
1. e2e4  1... e7e5  2. f1c4  2... g8f6  3. d2d3  3... c7c6  4. g1f3  4... d7d5  5. c4b3  5... a7a5  6. a2a4  6... f8b4  7. c2c3  7... b4d6  8. e4d5  8... c6d5  9. c1g5  9... c8e6  10. b1a3 This is the idea behind ​​White's play. The knight finds a good square on b5. 10... b8c6  11. a3b5 