Explanation for myself.

https://kernelnewbies.org/Linux_4.14#Asynchronous_buffered_I.2FO_support

"In this release, the preadv2(2) syscall with RWF_NONBLOCK will let userspace applications bypass enqueuing operation in the threadpool if it's already available in the pagecache."

For applications using a thread pool, like the aiofiles does for asyncio, preadv2() allows to bypass the thread pool which is obviously faster. If the read block, just uses thread pool as currently done.

Same rationale for pwritev2().