this transform is apparently designed to help in the detection of text within images.
While this is likely not very necessary for most books, it might help with books mixing images and text, to separate them.
It might also be useful to estimate the text area on a page.
From the paper at http://yoni.wexlers.org/papers/2010TextDetection.pdf:
The SWT converts the image from containing gray values to an array containing likely stroke widths for each pixel.
This information suffices for extracting the text by measuring the width variance in each component as [...] because text tends to maintain fixed stroke width.
This puts it apart from other image elements such as foliage.
Another interesting quote, from the end of the paper, emphasis by me:
The grouping of letters can be improved by considering
the directions of the recovered strokes. This may allow the
detection of curved text lines as well.