Differences between `emsdk install latest` and `emsdk install sdk-main-64bit`

I can understand most of the differences between the two, it's just basic directory layout stuff. What I'm confused by is the `libc++.a`, `libc++abi.a` and `libc++experimental.a` (along with all the libc++ headers). These are built for the host architecture (i.e. linux x64 or whatever), but building LLVM from source (`emsdk install llvm-git-main-64bit`) doesn't build/use/install libc++ for the host architecture.

```
$ readelf -h upstream/lib/libc++experimental.a | grep Machine
Machine: Advanced Micro Devices X86-64
Machine: Advanced Micro Devices X86-64
```

When would an end user need these in a binary release of emsdk? What am I missing? And how did they even get there, when `build_llvm` isn't passing `-DLLVM_ENABLE_RUNTIMES="libcxx"`?