You designed a page layout, a Photoshop photomontage or an imposition, using a provided document (ai/pdf or indd document, jpg/tiff/psd image, etc.) and now you are asked without any explanation to update (or redo) your work with a new version of the source document sent back to you, of course you will obey but you are too curious and you would like to detect what really are the differences between the previous and the new file.
If your new document has the same file size than the old, of course you'd better first check their MD5 or SHA1 checksums, or even compare every byte using the "cmp" unix command, sometimes people pull your leg and send you back the same file whose modification date changed only because the timestamps get lost during the mail transmission. But for example if the file was saved again by the author even without actual changes, it often will be not exactly identical to the previous one because something in the metadata or the compression is different.
In short, a simple check that you probably have ever done is to import and overlay the two documents in separate layers (optionally in difference mode) in Photoshop, Illustrator or here in InDesign, this is just what this script does (similar scripts may have been already written by others) and it intends to save you some seconds by avoiding you to do it manually (especially in the case of multi-pages PDFs). Without creating a new document, you launch this script in InDesign, it will prompt you to select the (two or more) files to compare (any graphic that can be placed in InDesign) and places every imported pages in a new document in as many layers as the number of files you selected, layers named from the name and modification date of each file, that you can now check and uncheck alternately to highlight the possible differences.