AssemblyScript is nice for small units of code that you might want to accelerate in a larger TypeScript project. The mental context switch is much less because the syntax and some of the tooling is the same, you only have to adjust for the semantics.

It isn't designed for or suitable for large projects.

The other reason to use it is that it's much closer to WASM than Rust or C++. It's the closest you can get to writing WASM without writing WASM.