You need it when a PDF has to go where PDFs don't fit: a presentation, a post, a form that only accepts images, a chat message where the preview matters more than the file.
Every page is redrawn at the resolution you choose and saved as JPG. High quality is right almost every time; Print produces larger, sharper images but also much heavier ones; Screen is for when size matters more than detail.
The images are numbered in page order and collected in a ZIP, so you download everything at once.