sscanf() is an example of a function with out parameters.
The function reads tokens from a character string and
converts them to internal representation according to
the types given in a format string -- it does the 
opposite of what printf() does.  

sscanf() needs out parameters for two reasons:
(1) it may need to return more than one value.
    The format string may indicate that any
    number of consecutive tokens should be converted
    and their values returned.
(2) even if only a single token is being converted,
    there needs to be some way to indicate success
    or failure.  C programmers use return codes,
    as C doesn't have exceptions.

Because C is purely call-by-value, the only way to 
get a value out via a parameter is for the parameter
to be a pointer.  The value of the pointer argument is
copied on the call.  Both the caller's pointer and the
callee's parameter pointer point to the same memory.
When the callee writes to that memory, the caller
can find the value written there on return.