The idea is to offer extensibility points in software by embedding their runtime into your current software. Say you have a database and instead of a DLS for querying, allow someone to upload a WebAssembly binary that has access to APIs you'd expose (`filter`, `map`, ...). I'm guessing they ease some of the complexity in moving data back and forth, their runtime is supported on N languages, and they use wasm's sandboxing properties.
The problem, for me, is even though the runtime supports many languages, the languages the plugins can be written in is limited. Rust and Go support wasm well, but AssemblyScript* is not JavaScript / TypeScript. If the idea is to extend software and make it more accessible, they need better support for interpreted languages: JS, Python, Ruby, ...
* - Closures are still not supported: https://github.com/AssemblyScript/assemblyscript/issues/798