In your print edition, blank pages are used to ensure that the following content begins on the right side of a page spread.
As an example, Vellum may insert a blank page before your first Chapter, which should always begin on the right:

To see how blank pages are used in your own book, we recommend viewing your generated PDF as spreads. You can do that by opening the PDF in the Preview app on your Mac and enabling View ▸ Two Pages.
Chapters
Vellum will always ensure that your first Chapter begins on the right. You can control the behavior for later chapters in Print Settings:

Vellum may insert a blank page before any chapter that is set to begin on the right.
Front and Back Matter
Most elements of front or back matter (like Also By and About the Author) default to beginning on the right. You can adjust this behavior using the Begin On control:

When an element is set to begin on Either Side of Spread, it will come right after the preceding element, without any blank page before it.
You can learn more about configuring Elements in Vellum Help.
Still have questions? Contact us.