The  main move in this reversed Slav defense type of position. Otherwise Black  wants to play Qd8-b6 himself.
1. d2d4  1... g8f6  2. c1f4  2... d7d5  3. e2e3  3... c7c5  4. g1f3  4... b8c6  5. b1d2  5... c8g4  6. c2c3  6... e7e6 