I've got a theory- thinking in words aids one's memory. By memory I don't just mean like "what did I come into this room for?" but I also mean that our minds use words as value-holding variables with which to construct complex thoughts and arguments so that we do not have to hold every detail of every element of our thoughts in instinctive memory. Because of this I also believe that having a large vocabulary increases one's intelligence and the potential complexity and logical soundness of one's thoughts and reasoning.

Edit: Perhaps a more clear way of explaining this is that one's vocabulary is like information stored on a hard drive and one's instinctive thoughts are like ram. Ram is much faster, but it is volatile (short-term) and has much less storage space for its cost.