Why iPhone videos use MOV format
iPhone video files often use a MOV container because it fits Apple camera and editing workflows.
The short answer
Apple has long used QuickTime-style media containers in its camera and editing tools. The MOV extension fits that ecosystem and can store video, audio, timing information, and metadata from the camera app.
Why sharing can be awkward
A MOV from an iPhone may be perfectly valid, but the person receiving it might open it on a browser, Android device, Windows app, or school platform that expects MP4. Conversion is useful when the destination cares about the extension or the codec.
What changes when you convert
Converting to MP4 does not improve the original recording. It makes the package and, when needed, the encoded streams more likely to play elsewhere. Quality depends on the export settings and whether re-encoding is required.