I built a testing framework, and I wanted it to generate HTML reports during testing with not post-processing report compilation step. I wanted the html in real-time so if a test was cut short for any reason from killing the job to power failure, you'd have a readable html report showing where things stopped.
I could do this by just appending divs as rows without closing any of the parent divs, body or html tags.
So the more general answer, anytime you want to continuously stream html and not want to wait until the end of the document to begin rendering.