These essentially serve as APIs for abstractions like Process Creation.
Why?
Why would we build such an interface to do what should be the simple act of creating a new process? Well, as it turns out, the separation of fork()and exec() is essential in building a UNIX shell, because it lets the shell run code after the call to fork() but before the call to exec(); this code can alter the environment of the about-to-be-run program, and thus enables a variety of interesting features to be readily built.
The shell is just an user program, which shows you a prompt and then waits for you to type something into it.