They also help fanboys go BAAAAAAAAAW.
Like Kainmaster.
But in all seriousness, Sirius nailed it.
If one wanted to use Sophia, for example and she turned out good, that's fine. After all, no one is attempting to keep another from playing like that. But that's personal experience that will not necessarily apply to others. Hence, the averages work the best, etc.
They help form logical suggestions, should a new player ask which characters to use.