If Bf5 is played instead, Qb3 becomes problematic, cxd5 cxd5 Qb3. Qb6 Nxd5! wins a pawn, and b6 e4 dxe4 Bb5+ Nd7 Ne5 is no better. Best is cxd5 Nxd5 Qb3 Nxc3 bxc3 with a comfy position for white.
Compared to the QGA, the move order forces white to play a4 after dxc4 to secure the pawn.
I may be showing off my profound lack of positional understanding here, but in many lines of the slav black ends up taking the c pawn, often on move 4. I don't understand this. If black plans to take the pawn, why protect the d5 square with c6 first? Why not just play a QGA? Please enlighten my darkness. I find I am somewhat drawn to the slav defense as a response to d4 and c4, but I think I am missing the basic point.