Hey SpinKube maintainer here. You're right that you could just run your Wasm (or Spin app) directly in a container and there often are reasons you might want to do that. But, SpinKube executes WebAssembly using a containerd-shim which means we avoid the overhead of starting a container and rather just directly execute the Wasm.
https://www.spinkube.dev/blog/2024/03/13/introducing-spinkub... is a really good blog post on Spin that talks about this.