It can be nice to have a bit of space between chapters in a book, or major sections in a shorter publication.

If  you use page breaks in MS word to separate chapters, some formats will strip out these pagebreaks.

To work around this,  put an extra paragraph break before and after the page break so some separations will persist.

But make sure, especially for Smashwords, that you do not put more than 3 paragraph breaks in a row, or you will get an error message. This is because too many breaks can produce blank pages in the final ebook, annoying your readers.