They've not bought him for the sake of it though. It was clearly a Diaz type scenario where Liverpool have wanted him and there was interest in him from other clubs so Liverpool have decided to move before he ends up elsewhere. The injuries to the forward line will have also impacted the decision to do it this month.
Liverpool currently have about 9 players who can play in midfield. Yes we all know the arguments about the qualities, or lack of, of certain individuals but there's a few of them out of contract in the summer. Needing to move some of them on, coupled with the player they want not being available in January is probably why we won't see a midfielder arriving this month.