A line edit typically occurs between developmental editing and copyediting, and it’s a sort of transition between making large structural changes and perfecting the mechanics of the language. Not every manuscript needs a line edit, but a line edit would likely enhance most manuscripts because its primary focus is not on perfecting the language but on elevating it.
I sat next to a woman on a cross-country flight who read Fifty Shades of Grey from cover to cover with such rapt attention that she didn’t respond to the flight attendant. When we landed, the woman looked confused and not too happy to step back into real life. She collected her things with the glazed, glassy eyes of the fictive spell.