Return sends data outside of the function, to whatever function was calling it.
Returning the parent sends the parent data outside of the function.
For example, vectorAdd("1 1 1", "1 1 2"); would, if you replaced all variables with their values, look like this at the end:
return "2 2 3";
If you packaged vectorAdd, it's not going to be able to send that "2 2 3" unless you also return it's value.