In page preview mode, or print preview mode as Word now calls it, you can see all the pages of your document and how they should look when they are printed out. This could also be referring to the document you see while you are editing it if you don’t mess with Word’s default page layout. Either way, you can see all the pages in a document - even if it’s up in the 600 page long range.
This is helpful for editing work, for printing, and for general viewing on the computer. It also lets you see problem spots or blank pages that shouldn’t be there. That’s really all it’s there for, but it’s one of the most helpful features of this program.