Why is 5. Bd3 considered a strong move against the Kan variation of the Sicilian Defense:
1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 a6 5. Bd3
You very rarely see the Bd3 move played against any other variation of the Sicilian...
The difference is that in the Kan white exchanges the knight on c6. In other sicilians this would be a concession, but here black has already played a6, which will not be particular useful after Nxc6 bxc6. In the other sicilians, if white doesn't exchange the Nc6, it will often go to e5 and threaten to exchange a Bd3, that's why the bishop often isn't well placed there. The plan connected to Bd3 is of course to play e5 sooner or later.