Use this option with headings to prevent them from starting at the bottom of one page and continuing at the top of the next. That looks bad.
Like widow and orphan control, keeping lines together will create gaps at the bottom of pages. But unlike widow and orphan control, you only want to keep lines together in special situations, not as part of your default text formatting.
Why? Keeping lines together is a blunter technique. It only works on whole paragraphs, so the longer the paragraph, the bigger the gap.
If you need to make groups of elements stick together, the keep-lines-together option works well with hard line breaks. Recall that hard line breaks don’t create a new paragraph, but rather a set of lines. Keeping lines together will ensure this set of lines appears on a single page.
For instance, it’s helpful to keep lines together in signature blocks:
|May 19, 2013|
Boxer Bedley & Ball
Here, you’d put a hard line break at the end of each line. Then you’d set the whole signature block—which is a single paragraph—to keep lines together. That way, you’ll never have to worry that half the block will end up on one page and half on the next. All of it travels together.