The default margin for a Google document is 1 inch of white space around the text on a page.

Documents have margins so that the document looks pleasant and is easy to read.

Margins also help make sure printers can print the text, since many printers can't print text that goes all the way to the edge of the page.

Using Page Preview to check the margins in a document before you print helps you make sure the page looks nice and that all the text in the document will show on the printed page.