So people new to the series frequently ask me what a companion is for. The question usually has pervy undertones. Not surprising, but no, although there are several interlacing reasons, the pervy ones are not among them.
Not that there’s never any sexual or romantic tension, but it gets uncomfortable when there is. Like the man says, he needs a mate, not to mate.
At it’s most practical, the companion is a storytelling device to help with exposition. Monologues can get tedious, but if there are at least two people, they can reveal facts in more dynamic ways than if there’s just one. One person can explain things to the other, or they can bounce ideas off each other, or consult each other, or argue, or whatever. Their communication informs the viewer in a more natural manner than a voice-over or monologue could.
Especially, the companion is the stand-in for us, the viewers, ourselves. Usually the companion is someone from our world or one of similar sophistication, so that the communication isn’t over our heads. We get to discover the Universe through their eyes because they are like us.
From within the story, the companion’s first purpose is to keep The Doctor company. Wandering can be lonely, and wonders are more wondrous when shared. Better with two, Rose said. Among his people, when his people were still about, he was pretty much unique already. He’s got all their arrogance but was born without the required stick up his butt. He failed to acquire their prejudices. So he was already alone. It helps to find people who understand him, if only a little. Humans tend to be a bit better at this than most.
Then, later, Donna put her finger on the best reason to have a companion. Someone has to stop him. His wrath can get the better of him, much more so in these later years, and he can do terrible, terrible things in the grip of a righteous fury. Without another mind, another voice to help give a sanity check, what might he do? His people are no longer there to keep an eye on him. He can take time into his own hands, the Time Lord Victorious. He’s only one hideous slip from madness. Or possibly evil, if they’re different things, and they may not be.
Cackling, that’s it. Terry Pratchett explains in the Discworld novels that witches have to be careful not to isolate, to keep an eye on each other, because they’re vulnerable to a condition called cackling. This doesn’t mean their laughter specifically but the tendency to think that because they’re more powerful than others, they’re better than others, or above societal rules, or the authors of right and wrong rather than adherents. It’s the state of mind that leads to gingerbread houses and poisoned apples. So they do a lot of visiting and socializing and other activities to help stay grounded and occasionally humbled. I bet the Time Lords did something themselves to mitigate the effects of too much power. Usually.
Companions can help prevent cackling. Usually. That’s where it’s gone a bit haywire in the Demon’s Run storyline. Because the companions are the victims swept up into this mess, they can’t really help sanity check him. They’re too busy fighting for the safety of their family. It’s left to River to do this. And we still don’t know (but I hope we find out this series) just what The Doctor did to cause a full-tilt war against him. But if River’s lesson for him was supposed to get him to ease up, it must-have-been-going-to-be one helluva doozy in the first place. My, tenses are difficult.
That’s an aside. Companions are useful, and not just for the usually-mentioned purpose of moving the plot along by having some adversary abduct them or invade their brains or something. They do that, but it’s not the whole point. It’s a demanding job all around. Don’t you wish it were yours, though?