What webassembly provides is a bytecode format that multiple languages can target, that isn't owned by a corporation, and that's intended (as per their own docs[0]) to run both natively and on the web (with browser vendors already opting in.) This offers benefits over native code tied to an operating system, existing bytecodes and VMs tied to a specific language or company, and allows the possibility for applications to seamlessly operate on the web and offline.

The only thing a browser offers is an HTML and Javascript context... which are necessary to run it in a webpage, but not really the entire purpose of webassembly as a concept.

[0]https://webassembly.org/docs/non-web/