People can use software written in languages that need a compile target. For example you can write web applications that use libraries written in C/C++ that before would have needed to be hand ported to JS or compiled using technology like asmjs that was creaking at the seams. From the user perspective it enables more rich web applications.

From a performance perspective it's also a different target for web engines so has different opportunities for optimization. It's not clear to me we've reached the limit on that at the moment, or how much work browser developers are putting in to optimizing their WASM codepaths. The standard also includes support for things like SIMD which is not natively available in JS land which should provice a boost for suitable workloads and there is a lot coming through the proposal pipeline (https://github.com/WebAssembly/proposals) that will also let WASM diverge more from JS.

From a developer perspective allowing more languages than JS also has some productivity benefits. Both in using languages they are more familiar with and those better suited to certain domains. For example you can write math code in JS, in particular making use of TypedArrays to help things along but its much more straightforward in a language that supports more 'native' types.

Because Javascript would need to change a lot to better support compiled languages (for instance adding 'proper' separate integer and float types, but that's just the tip of the iceberg). This means extending the ECMAScript standard with features only few web developers care about, increased complexity in JS engines, and potential design-conflicts for new JS features between 'compile-friendly' vs 'human-friendly'. With WASM, the WASM peeps can focus on WASM and the JS peeps can focus on JS.

Speed is not fine. It's good enough for a lot of apps, but it doesn't get the job done for anything performance sensitive. An obvious issue is the lack of real machine number types, which makes anything numeric much slower than it could be.

Furthermore, JS semantics are limiting in other respects. E.g. there are people looking at tail calls and continuations in WebAssembly, which I can't see ever coming to JS.

I don't think speed is fine. Here's a write-up from Mozilla which talks about the performance benefits of WASM compared to asm.js: https://hacks.mozilla.org/2017/03/why-webassembly-is-faster-...