Cutter John wrote:
Why the need for a modern companion? By now we all know how the DW universe works. We don't need a companion to handhold us through it anymore.
It's nice for the Doctor to have someone to talk to. Indeed, that is one of the primary purposes of a companion according to RTD.
Of course, back in the 70s Tom Baker insisted he didn't need a companion, and we even got a companion-less adventure just to shut him up on the matter. Although The Deadly Assassin is generally well regarded, I'm not sure how much longer they could have kept that up. Take note, part 3 has practically no dialogue at all.