ENGINE ECU QUESTION(S)

Hello all
Does anybody know if you purchase a replacement ecu with EXACTLY the same part number and codes as the one fitted to your car DOES the replacement unit STILL have to be reprogramed to function properly,-I can understand a different one with a different part number needing to be re-coded, but would assume that one with exactly the same part number would have been set up to the same parameters as the one currently fitted! anyone enlighten me, please!.

Also whilst looking for the part number on my current ECU , having removed the plastic cover I found at the back of the wiring loom near the bulkhead what can only be described as a decomposing mass of what looked like a sponge filled with glue which was breaking down!- does anybody know what this is/was or why its there and what it's for, just looks to be making a mess now b ut would be interested to know if it had a function at some time?.
#thanks all
regards Steve