It's because all parameters are passed by value in C. If you call a function with a pointer parameter you may change that pointer in the function but this will have no effect on the caller's value of the pointer.
If you need to communicate a modified pointer back to the caller, returning the modified pointer is the way to go. Note that this also only passes a value back.
Another way would be to pass the address of a pointer (i.e. a pointer to a pointer). The called function can then store a new pointer value at the given address.
