Is there any real-world example where you would need this kind of nonblocking behaviour?

All I can came up is some code that communicates with thousands of ssh servers at once. Looks very unlikely to me....

int SFtp::Get(SFtpHandle handle, void *ptr_, int size0)
{
	int done = 0;                             // <- Can't be used in non-blocking mode.
	char *ptr = (char *)ptr_;
	Cmd(SFTP_START, [=, &done]() mutable {    
		int size = size0;
		if(OpCode() == SFTP_START) {
			if(FStat(HANDLE(handle), *sftp->finfo, false) <= 0) OpCode() = SFTP_GET; // <- This is for higher-level api. should be removed.
			else return false;
		}
		while(size) {
			int rc = libssh2_sftp_read(HANDLE(handle), ptr, min(size, ssh->chunk_size));
			if(rc < 0) {
				if(!WouldBlock(rc))
					SetError(rc);
				return false;
			}
			else {
				if(rc == 0)
					break;
				size -= rc;
				done += rc;
				if(WhenProgress(done, size0))
					SetError(-1, "Read aborted.");
				ssh->start_time = msecs();
			}
		}
		LLOG(Format("%d of %d bytes successfully read.", done, size0));
		return true;
	});
	return done;
}