A natural reaction, fixing the queenside. The b4-square is weakened, but White has an idea:
1. g1f3  1... d7d5  2. g2g3  2... g7g6  3. f1g2  3... f8g7  4. d2d4  4... g8f6  5. c2c4  5... c7c6  6. e1g1  6... e8g8  7. a2a4 Only the eighth most popular move in this well-known position.