Comment by khaledh

From "Operating Systems: Three Easy Pieces" chapter on "Process API" (section 5.4 "Why? Motivating The API") [1]:

    ... 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 separation of fork() and exec() allows the shell to do a whole bunch of
    useful things rather easily. For example:

      prompt> wc p3.c > newfile.txt
    
    In the example above, the output of the program wc is redirected into the output
    file newfile.txt (the greater-than sign is how said redirection is indicated).
    The way the shell accomplishes this task is quite simple: when the child is
    created, before calling exec(), the shell closes standard output and opens the
    file newfile.txt. By doing so, any output from the soon-to-be-running program wc
    are sent to the file instead of the screen.

[1] https://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf