The page-break-before option is intended to be incorporated into paragraph and character styles so all paragraphs of a particular style will start at the top of a new page. For instance, you might apply it to your top-level heading style. In a long document, typing hard page breaks in front of each heading would be tedious.
A small wrinkle arises when you use page break before with headings that have space above and below. Your word processor, trying to be helpful, will ignore the space-above setting if the heading is the first thing on the page. If that’s what you want, great. But if it’s not, you’ll have to devise a suitable workaround.