When you actually need this
Posting a certificate on Instagram. Sending one page of a contract in a chat. Pulling a diagram out of a report to use in a presentation. In each case the recipient wants an image, not a document.
Quality and resolution
A PDF page has no fixed pixel size — it is rendered at whatever resolution you request. Higher settings produce sharper images and larger files. If the PDF contains a low-quality scan, no setting will make it sharper: the detail simply is not there to begin with.
The reverse direction
Going the other way — images into a single PDF — is just as common. People photograph documents with a phone and need to submit them as one file. That is a separate tool, but the same idea.