System calls are actually kernel functionality that can be invoked from userspace via glibc stub routines. They serve a critical function; they connect userspace to kernel-space. If a user program wants to request something of the kernel (read from a file, write to the network, change a file's permissions), it does so by issuing a system call. Therefore, system calls are the only legal entry point to the kernel. There is no other way for a user-space process to invoke the kernel.
For a list of all the available Linux system calls, see section 2 of the man pages (https://linux.die.net/man/2/). One can also do: man 2 syscalls to see the man page on all supported system calls
Another way to think of this: the Linux kernel internally ...
