Generate as much placeholder text as your mockup needs, measured in paragraphs, sentences, words or characters. Output as plain text or wrapped in paragraph tags, and choose whether to open with the traditional "Lorem ipsum dolor sit amet".
It is scrambled Latin from Cicero's De Finibus Bonorum et Malorum, written in 45 BC. Typesetters have used it since the 1500s because text that is nearly-but-not-quite readable lets people judge layout without being distracted by the content.
Because reviewers read it. Placeholder text keeps attention on spacing, hierarchy and rhythm. The counterargument is that lorem ipsum hides how real content behaves, so it is worth testing with realistic copy before you ship.