If your document is going to be duplex printed, you almost certainly want an Odd-Page section break so that the new chapter will start on a page on the right. It seems that this is what you are likely using. This blank page does not show up in Print Layout View but will show in Print Preview and will show up in the page numbering.

The Next Page section break will not insert a blank page but will start a new page.

• Odd-numbered-page Section Break in Word
• Working with sections by Dave Rado, MVP