When you need pages as images
Slides that only accept images, chat apps that preview pictures but not PDFs, web pages, print shops, OCR pipelines, social posts of a menu or poster — plenty of destinations want an image, not a document. This converter renders each page with pdf.js, the same engine Firefox uses to display PDFs, so what you download is exactly what the page looks like — fonts, vectors and images included.
Choosing resolution honestly
A PDF page has no pixels until it is rendered, so resolution is your choice. At 150 DPI an A4 page becomes roughly 1240×1754 px — sharp on any screen and the right default. 300 DPI doubles that in each direction for print or deep zooming, at about four times the file size. If the images are headed somewhere with a size limit, convert at 150 DPI and run the results through the image compressor.
Related jobs
Going the other way — images into a PDF — is Image to PDF. Shrinking the whole document while keeping it a PDF is Compress PDF. Pulling out pages as PDF pages (text intact) is Split PDF.