Here I think we have one of the more fascinating interactions that occurs in the mind. While these two facilities are certainly not the same, it is hard to imagine the point of one without the other.
In order to use the intellect we have to imagine the propositions we are considering, whether they be evidently true or hypothetical.
The imagination, which is so free to run wild with possibilities, produces its best work when restrained a little by the rational boundaries of what might be possible.
I know that my best creative acts come when I have reduced the possibilities for my imagination to work on to a tightly constrained set of boundaries - under these conditions it goes wild to come up with something as far fetched as possible without "breaking the rules".
The art of thinking likewise would starve if it were not allowed to imagine the limited or endless possibilities of a given thought.