Game engines are huge behemoths, and contain algorithms for handling everything and the kitchen sink. While that's normally good, if you want to do something small, or possibly do something outside the bounds of a normal game enine, you suddenly have to learn how to do everything yourself.
Or perhaps you just like the "type it and run" style of development for shadertoy shaders and want something similar that you can host on your system as (pretty much) a single file.
Either way, this book chronicles my adventures into creating WASM games from scratch in Rust. All the code is available on github.