When you actually need to convert JPG to PDF
Most government and corporate portals will not accept a JPG when they ask for a document. They expect a PDF, even if the underlying content is a photograph. So converting JPG to PDF is required even when the form technically displays just an image.
Other times you simply want a tidier deliverable: a single PDF is easier to email than a ZIP of photographs, easier to print as a contiguous document, and easier to archive. Common cases include combining a phone-photographed signed agreement into one PDF, bundling scanned receipts for a tax filing, attaching a portfolio of design work to a job application, or building an ID document set with passport, address proof, and signature on separate pages.