Right now, the team who is seeing the most success with Bevy are actually building CAD software! They needed something powerful, fast, incredibly flexible with "game-like" functionality.
That said, from talking to the hobbyists in the community, they like Bevy because it's:
- code-first
- written in Rust (C# is fine, but Unity's C# is something of a DSL, and game engine C++ is really rough)
- cares about good documentation and a helpful learning experience
- supports them in contributing fixes to issues they run into
- is incredibly flexible: you can just swap in your own physics or rendering to realize that weird idea you had, or just for the learning experience
Someone liked the idea of writing a game engine in Rust and so they did. I wrote my own sidescroller engine when I was 16 as well, although there were existing things around and it thought me a great deal about how to tackle multiple classes of problems.
Now the bevy devs seem way more experienced and if there was already a game engine that ticked all their boxes they might not have started developing a new one. But the point here is: Not everything is a product with an clearcut target audience — and not everything has to be.
Projects like these ultimately help the whole field of game engines to move on and innovate.