horacethegrey
First Post
Plus they help keep the program watchable even if the Doctor is turning audiences off. I remembered thinking that Catherine Tate's performance as Donna was the only thing that kept me watching Season 4, after Tennant's 10th Doctor suddenly decided to become an unlikable prat.That's been covered time and time again to the point of repetitive nausea. He goes mad without companions. They ground him and keep him from doing bad things. This has been an ongoing theme since the 2005 reboot - from Donna getting him to stop murdering the red spider babies to him going all "Time Lord Victorious". It's referenced a couple of times every season. They're his conscience, almost.
Going further back to the series, after Lis Sladen (God Bless Her) left the TARDIS, Tom Baker thought the Doctor didn't need companions and preferred him to travel alone. The showrunners weren't having that though, so Tom decided to make his costar's lives hell (Note though that he's since apologized to many of them for his behavior, particularly to Louise Jameson).