It offers a preview of the document. This button is called Print Preview, and it would be found in the document software Microsoft Word. Although how you see the document on the screen is how it will print, there are times where it will print incorrectly.
A header could be missing or shifted, or the page numbers could have disappeared. This is why the print preview is essential. Pressing this button will allow you to see the document exactly how it will print and fix any errors you may see before printing the document out.