Why iPhones default to a format that causes this problem
MOV is Apple's own container format, and it's deeply integrated into iOS and macOS — which is exactly why it works flawlessly within Apple's ecosystem and less reliably outside it. Windows Media Player, many non-Apple video editors, and various web platforms have inconsistent or missing support for MOV, especially depending on which specific codec is used inside the container.
Since MOV is the iPhone's default recording format, this becomes a recurring friction point for anyone who records on an iPhone but needs to edit, upload, or share on non-Apple software and platforms — which describes a huge share of everyday video sharing.
The HEVC complication specific to newer iPhones
Newer iPhones often encode video inside the MOV container using HEVC (H.265) rather than the older H.264, since HEVC compresses more efficiently at the same quality — but HEVC support is considerably less universal than H.264's, and some older software and devices can't decode it at all. This means a MOV file from a recent iPhone can fail to open somewhere that an older MOV file (using H.264) would have played fine, which is a common source of confusion since both files share the same MOV extension.
Converting to MP4 with H.264 specifically (rather than just changing the container while keeping HEVC) resolves this compatibility gap completely, at the cost of the file being somewhat larger than the more efficient HEVC-encoded original.
What actually changes during conversion
This is a genuine re-encode of the video and audio streams, not just a container swap, so there's a small quality trade-off inherent to any lossy-to-lossy conversion — at a reasonable quality setting, this is not something you'll notice in normal viewing. Because it's a real encode, processing time scales with the video's length and resolution, same as any other format conversion.
When you don't actually need to convert
If you're staying entirely within Apple's ecosystem — editing in iMovie or Final Cut, sharing via AirDrop or iMessage, playing on another Apple device — MOV works natively without any conversion needed, and converting unnecessarily just adds an extra processing step and a small quality cost for no real benefit. Conversion becomes worthwhile specifically at the point where the video needs to leave that ecosystem: uploading to a non-Apple platform, opening in Windows software, or sharing with someone on a non-Apple device that's having trouble with the original file.