What's the importance of image at infinity?

I'm working on a project about stereoscopes (take two slightly offset images, when you view them both through converging lenses they form one image which has a feeling of depth). In stereoscopes, the focal length of the lens is the same distance as the object (the two images), producing an image at infinity. What is the importance of this? (Why must the image be at infinity? And how is this related to optics? Does image at infinity allow the perception of "depth"?