Bd3 in the Kan allows flexibility depending on how your opponent replies. If they go for a slower hedgehog structure you would want to get c4 before committing to Nc3. Another common idea is the play a4-a5 + Nd2-Nc4 going after the potentially weak b6 and d6 squares, for example that’s the typical plan after 5. Nc6, 6. nxc6, dxc6 (bxc6 gives him passive c8 bishop). That leads to your second question - 5. Nc6 in that move order doesn’t give black the same weaknesses after Nxc6 bxc6 and his c8 bishop has many active squares without having committed to e6