Using C bindings without fear would be a big one. If you use NIFs in Elixir you may bring down the whole VM with a segmentation fault. While Lunatic limits this failure to only the affected process. WASM was designed to be run inside the browser so the sandboxing capabilities are much stronger.

Another would be raw compute power, WASM comes close to native in many cases.

Other advantages over BEAM: Static typing, multiple languages with a WASM target available, live migration of process to the browser.