Lorem ipsum is a promise to deal with the hard part later. It lets a page look finished before anyone has decided what it should say. We have learned, slowly and occasionally the hard way, that later is the most expensive time to find out the words do not fit.
The words are the design
When you lay out a page around real sentences, the design starts making honest decisions. A heading that is actually three lines long stops pretending to be one. A feature with nothing true to say about it quietly asks to be removed. The layout bends to the meaning instead of the other way round.
So we write first. Rough, plain, unglamorous drafts — the real claims a business can stand behind — and only then do we start moving things around. The page almost always comes out better for it, and the client almost always ends up saying something truer than the placeholder would have let them.