Think about a worksheet or an exam paper, where most of the questions have one text frame and one music frame, but some questions might use more frames (for example there might be one music frame for the question, and another one with blank staves for the student’s answer).
If you change the first question, on the sheet , you don’t want Dorico to try to reformat all the other questions for you, though you might have to reformat some of them yourself to get sensible page breaks etc (You probably wouldn’t want the start of a question on one page and the end on the next page, etc).