GIFs are a smaller files I can upload much easier opposed to an MP4.
MP4 videos can be literally 10x smaller than a GIF and better quality, so a lot easier to upload on a slow connection.
Also there's animated emojis that seem to use GIFs on Slack, Twitch, Discord etc...
I tried adding a GIF on Discord and... it's an MP4 video again.
It's always in website owner's interest to use and support videos. They are literally 10x more efficient while being better quality. It can substantially reduce hosting costs by reducing bandwidth usage so much. As I said, these days everywhere you see something labelled "GIF", it's usually actually a video.
I'd be interested to hear any cases where some software or service accepts a .gif file but not a video.