HEIC to JPG — iPhone photo → cross-platform sharing
HEIC → JPG is the "share this iPhone photo with a Windows user / older email client / print lab" conversion. HEIC is the iPhone camera default since iOS 11, but Windows 10, many web forms, and older Android versions can't open it. Converting to JPG produces a ~2× larger file at equivalent quality (HEIC is that much more efficient) — worth it for compatibility.