Shape constancy provides redundancy in the reading process.

Shape constancy is good for decoding worldly objects, although it wreaks havoc when we try to draw something, like a table. If we draw what we perceive (a rectangle), it simply doesn't look right. It takes someone with training in art to learn to ignore the percept and draw the retinal version of the tabletop.
Shape constancy also helps us to recognize letters or words when they get twisted or turned.
(Please look at the graphic version for a second)
This is tricky, though, because many of our letters take on a different meaning, depending upon their position in space.