I am trying to use a single word/simple sentence to explain a situation where adding/not adding a part to a device won't make any difference to its functionality.
irrelevant - adjective:
Not connected with or relevant to something.
[Source: Oxford Dictionaries online]
Of no significance
From the ODO:
Of no value or importance: ‘a nugatory and pointless observation’
Here's a software-related example:
The process of analyzing the impact of change requests can take so long and be so extensive that it has a destabilizing effect on the main team: they are only too aware that their current work may be rendered nugatory following the approval of the change requests.