Why were M C Escher's works important?

I'm not necessarily sure that "important" is quite the right Word. His work is unusual and well-liked by most mathematicians and scientists, but ultimately, they're still "just drawings", not a cure for cancer or anything.

However, Escher's art often can be useful in illustrating mathematical or physical or philosophical concepts, even if it's just to the extent of pointing out why some of his drawings couldn't be constructed in real life. Recursion, tesselation, and impossible figures are common Escher subjects.