I'm a firm believer in teams having a good spine. Van Dijk, Keita and a proper, dominant goalkeeper goes a long way to having that.
There's also the way a central defensive partnership defends that improves a defence. It's not just the defender's strengths in isolation, as I see so many people solely take into account.
It's about the angles they give away and how they pressure, how they mark and read, how far they push you out to the left or right etc...
Van Dijk and A.N.other will markedly improve our defense. Currently, our defenders- while allowing a low amount of shots on goal- make sure those chances are quality chances for the opposition and that is not good at all on any goalie.
I've watched some footage of him and one thing I've noticed is that he gives A LOT of instructions during defensive transitions.