Another use-case is when you are writing threaded code that uses the clone() syscall instead of pthreads, usually for something with high performance, unusual clone flags, or a very small stack.

Most libc functions, including the syscall wrappers and all pthreads functions, aren't safe to call in threads created by raw clone(). Anything that reads or writes errno, for example, is not safe.

I've had to do this a couple of times. One a long time ago was an audio mixing real-time thread for a video game, which had to keep the audio device fed with low-latency frames for sound effects. In those days, pthreads wasn't good enough. For talking to the audio device, we had to use the Linux syscall wrapper macros, which have been replaced by nolibc now. More recently, a thread pool for high-performance storage I/O, which ran slightly faster than io_uring, and ran well on older kernels and ones with io_uring disabled for security.