Not grasping concept

["Tim Kredlo" <TKredlo@xxxxxxxxxxxxxxxx>]

Please help!

I am having a VERY difficult time grasping the concepts behind
prototypes/return values/procedure interfaces.
Here is what I understand the recommended process to be:

Define a prototype for the subprocedure (TESTSP). and store it in the
source PROTSP.

It will return a numeric field 15 characters long with 3 decimals
 and it requires 3 numeric input parameters.
        D TestSP        PR            15S 3
        D Unit                             2S 0
        D Qnty                            8S 0
        D Long                            3S 0

So far, so good.

Now I am going to write the subprocedure itself. 
        H NOMAIN
        // Include the prototype
         /Copy PRODLIB/QRPGSRC/PROTSP
Still OK. BUT then:
        // Define the procedure interface
        D TestSP      PI
        D Unit                             1A
        D Qnty                            8S 0
        D Long                            3S 0
Why is this necessary?
Didn't the included prototype already declare these fields?
Wasn't that the purpose of the prototype?

        // Declare the field to be returned:
        D Footage                     15S 3
Why is this necessary?
Didn't the prototype already declare the return value
type/length/decimals?
        
        /Free
        //Calculate the return value
        Footage = Unit * Qnty * Long; 
        // Return the calculated value
        Return Footage;
        /End-Free

I must be missing some very basic concepts, because I would have thought
that the following was ALL that SHOULD be necessary for subprocedure
TestSP:

        H NOMAIN
        // Include the prototype
        /Copy PRODLIB/QRPGSRC/PROTSP
        // Calculate and return value
        /Free
         TestSP = Unit * Qnty * Long;
        /End-Free

No PI defined - already did that in the prototype.
No "Return" variable defined - use the subprocedure name. Type, etc.
defined on prototype PR line.
No Return statement required - stated in prototype that the subprocedure
returns a 15S 3 variable.

Please - what am I missing?
Where can I go to get an explanation as to WHY I need to add
what I consider (obviously incorrectly) unnecessary coding?

OR: Am I coding things here that ARE unnecessary?

Thanks in advance for any help.

Frustrated, uninformed, and confused (but only in my mind),

Tim Kredlo