Firecracker is a fine technology, but serverless companies have started taking advantage Wasm's faster start-up and invocation times for use cases of running Wasm on the server (https://www.youtube.com/watch?v=yqgCxhPAao0). The deny by default security policy makes Wasm a popular choice to run code in isolation, particularly for maximizing hardware resources in the multi-tenant environments these serverless companies operate.
> Is there a good reason to do this?
One use case to run JS inside a Wasm VM is Shopify Functions. Shopify allows their customers to customize things like checkout flow by writing code compiled to Wasm which gets executed during the checkout process. They want their customers to be able to write JS as well as other languages. https://github.com/Shopify/function-runner
> I thought WASM typically used the V8 JavaScript interpreter as its sandbox and to execute code.
V8 is popular for running Wasm on the web and for some serverless companies, but there are a bunch of serverless, blockchain, and iot projects that use other Wasm runtimes (Wasmtime, WAMR, WasmEdge, and Wasmer to name a few) - https://github.com/appcypher/awesome-wasm-runtimes
> Is there a good reason to do this?
One use case to run JS inside a Wasm VM is Shopify Functions. Shopify allows their customers to customize things like checkout flow by writing code compiled to Wasm which gets executed during the checkout process. They want their customers to be able to write JS as well as other languages. https://github.com/Shopify/function-runner
> I thought WASM typically used the V8 JavaScript interpreter as its sandbox and to execute code.
V8 is popular for running Wasm on the web and for some serverless companies, but there are a bunch of serverless, blockchain, and iot projects that use other Wasm runtimes (Wasmtime, WAMR, WasmEdge, and Wasmer to name a few) - https://github.com/appcypher/awesome-wasm-runtimes