I think the initial reason for Lottie was to actually get AE animations to web. AE is very popular among designers to create some avant-garde UI ideas. The problem was 99% of them were absolutely impossible to implement with existing tech. Lottie was one of the steps to make at least some simple stuff real.

By avant-garde stuff I mean for example this guy’s work, he’s a killer: https://dribbble.com/glebich