Standardizing WASI: A system interface to run WebAssembly outside the web
WebAssembly is an assembly language for a conceptual machine, not a physical one. This is why it can be run across a variety of different machine architectures.
Just as WebAssembly is an assembly language for a conceptual machine, WebAssembly needs a system interface for a conceptual operating system, not any single operating system. This way, it can be run across all different OSs.
This is what WASI is — a system interface for the WebAssembly platform.