You must have a very different experience of email than I do. That sounds absolutely terrible to me. Am I missing some reason that this would be useful, or is this just a case of wildly diverging preferences?
Our product (Interactly) is virtual video interactions, so our main value prop is being able to see real people respond.
In that context we figured it would be interesting to see if we could include a 5 second GIF intro right into the notification email; mostly out of curiosity just to see if it would technically be possible.
I was surprised to find out it was mostly feasible, but you're right that it doesn't make sense beyond the fact that it would be a cool gimmick.
The file size reduction was quite impressive for our use case (roughly 10x smaller.)
In the end we did decide that while putting 5 second video intros in emails is cool, a 200k image is still a bit much for mobile users.