The starting position is the following (including the two options 10...a6 and 10...Bf5)
1. c4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 g6 6. g3 Bg7 7. Bg2 O-O 8. O-O Nxd4 9. Qxd4 d6 10. Qd3 a6 (10...Bf5).
Why did white choose Qd1-d3? First of all, this prepares to increase the control over the d-file with Rf1-d1 or Ra1-d1. Second of all, this protects the c4-pawn against Bc8-e6. Next, white is planning Rf1-d1 and c4-c5 to attack the d6-pawn. If black places the Queen on c7, then white has Bc1-g5xf6 followed by Nc3-d5xf6, ruining the enemy pawn structure.
Black's Bc8 has to guard the b7-pawn. Another piece has to take over this role, if the Bc8 wants to move to a better square. Black could try Qd8-b6 to protect the b7-pawn and put pressure on the enemy b2-pawn. Yet, 10...Qb6 is met by 11.Nb5 followed by 12.Be3. Black also has the thematic Nf6-d7-c5 or -e5. The problem is that white might have time with Nc3-d5 and Bc1-g5, which would cause trouble for the e7-d6 pawn chain. Black can also try Qd8-a5-h5, planning a kingside attack with Bc8-h3. Yet, 10...Qa5 is also met with 11.Nb5. Thus, 10...a6 prepares 11...Qa5 or 11...Qb6.