You don't inherently need a libc for wasm. That is, libc is usually the API for the platform; wasm doesn't have such a thing really. Your wasm module says "hey, these are the functions I expose, and these are the functions that I am planning to call from the environment" and it's the host's job to wire that up properly. In the browser, that means when you instantiate a wasm module, you need to provide all the required functions, and then you can call the exposed functions from JS. Outside the browser, what things you can call and will be called is defined by the host.

A libc can be useful inside of wasm for existing C or C++ code (or whatever) that is built for another platform, so that you can more easily compile to wasm as well. But it's an option, not required.

You don't necessarily need it. You could have some math focused code you want to call from JS.