If you take math to merely be 1+2 or think mathematicians are some kind of walking calculators you'd be right, but they aren't. Tell me it doesn't take imagination to picture in your head a counter example to the converse of the mean value theorem. Proofs take a large amount of imagination and creative thinking. Most math isn't as simple as being given an equation and solving it using simple algebra, that's only what most people ever learn or have to use. Calculus is taught as plug and chug in high school even, when there is a great deal more to it.
"Our imagination is stretched to the utmost, not, as in fiction, to imagine things which are not really there, but just to comprehend those things which are there" - Feynman
Math requires the thorough understanding of many concepts and identities in order to prove things, and you won't thoroughly understand them if you can't imagine them in their entirety, probing for weak points and searching for the bits that are vital to functionality.
Many times it's useful to approach difficult problems by imagining the graph first. Or try to imagine the ridiculous multidimensional space you're working in
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http://harvardmagazine.com/2004/01/on-mathematical-imaginat.html
