MOV is Apple's native video container (the default when you record on an iPhone or export from Final Cut), but a lot of non-Apple software, older Android phones, and Windows tools either mishandle it or refuse to open it at all. Converting to MP4 fixes that near-universally, since virtually every platform, phone, and video site accepts MP4 uploads.
Often the same video in a different box
MOV is Apple's QuickTime container and MP4 is the ISO standard that grew directly out of it, which is why the two are structurally so similar. A MOV recorded by an iPhone usually holds H.264 or HEVC video and AAC audio, and those are precisely what an MP4 holds too. In that common case the conversion is largely a matter of repackaging the same streams into the container everything else expects.
That is why this particular conversion tends to be quick and why the picture quality does not visibly change: the encoded video itself is not being reinterpreted, it is being rehoused.
The case where it is not that simple
Footage out of a professional camera or an editing timeline is a different story. A MOV can carry ProRes, an intermediate codec built for editing rather than delivery, which trades enormous file sizes for the ability to survive repeated re-encoding. Those files must be genuinely re-encoded to become an MP4, which takes real time and is where a long clip will make a laptop work.
Either way the reason to convert is compatibility. MOV plays natively on Apple hardware and stumbles elsewhere: Windows without the right components, Android, older smart TVs, and a good number of upload forms that accept MP4 and nothing else.