Mike Yates leaves the show at the end of the eleventh season. His player, however, who has only been able to attend occasional sessions of late, finds that he will be able to game on a more regular basis from now on. The Doctor's player is pondering a change, too, and briefly considers a physically weaker, more intellectual persona for his next regeneration - something, in fact, not unlike his original character concept.
So Yates' player decides that the party needs another action hero, similar to Ben or Steven way back in the third and fourth seasons. Although the GM has largely given up on "defending the Earth from aliens" adventures by this point, he has a few UNIT stories planned for when the players of the Brigadier and Benton are able to make it. So, since he'll be an action hero, it makes sense for the new PC to come from UNIT, too. Having just played Yates, though, the player is keen for something a little bit different this time.
And so he creates Harry Sullivan. Although he's a military member of UNIT, he's a doctor, not a soldier - the British branch's Medical Officer, in fact. He's also one of only two UNIT military staff members that we see in the classic series who aren't from the British Army (the other is Corporal Bell). Instead, Harry is a Surgeon-Lieutenant in the Royal Navy Medical Service.