Figma is probably the biggest web application that uses it for the main feature they offer (the entire in-browser editor is C++ compiled to Wasm).

You can read more about how they use it here: https://www.figma.com/blog/webassembly-cut-figmas-load-time-...

> Because our product is written in C++, which can easily be compiled into WebAssembly, Figma is a perfect demonstration of this new format’s power. If you haven’t used Figma before, it’s a browser-based interface design tool with a powerful 2D WebGL rendering engine that supports very large documents.