Why is a template text recommended to be as long as possible?

Why is a template text recommended to be as long as possible?

Designer at work who don’t have any content for their product yet have the possibility to insert a dummy text into their design to judge on the arrangement of text on their site, on readability or on fonts and sizes. A dummy text is also helpful to present a design without content to a client to show how the text is going to look like without irritating the client by real texts.