I am under the impression that no-std code is primarily the concern of embedded systems that might not have much of an OS [1] at all, nevermind a suite of system calls to Linux.

[1] Like a typical RTOS, that may provide some basic communication primitives, thread creation, and a hardware abstraction layer (HAL).

That is nor necessarily the case. Embedded is a big user, but there are other reasons to use no_std, such as explicitly wanting to avoid heap allocations, or wanting greater control over the native APIs being called.

Also Wasm support (can be viewed as a kind of embedded) and better portability. Anything that helps one separate the platform and more precisely define the platform boundary the better.

Libc is necessary, but not good.