The original idea for netlink sockets (where the name comes from) is to be able to do some network packet processing in userspace. E.g. to do stuff like virus-scanning on TCP connections.

The situation there is exactly the opposite of a syscall, it is rather that the kernel calls into userspace to perform a helper function.

Communication with the socket is still read()/write()/..., so there are still syscalls. The userspace program will do a read() to get the next struct+packet out of the socket.

The modern, syscall-less interface for stuff would be io_uring. There, you do not need to read(), you can just get your data written into a userspace buffer that you can mwait or poll on.