@Xtropy To me the choice is enhance an existing building or build something new. The adjacent addon should occupy a tile.
Right, but the choice to enhance an existing structure or build something new has nothing to do with adjacency. This "choice" exists whether or not an "officer's quarters" is built on the other side of the city or directly adjacent.
I'm asking about the point of adjacency, not the choice as how to spend available tiles/resources.
If someone who builds a hypothetical officer's quarters is _always_ going to put it next to a barracks to gain whatever advantage the officer's quarters would get by being built next to the barracks, what is the point of requiring the adjacency, because when would someone build an officers quarters and not put it next to the barracks?
Off the top of my head I can think two ways the adjacency could add a real choice or decision,
1) A barracks can gain advantage from multiple kinds of "add-ons" but can only have a subset of those connected to it at any given time. For example, a barracks could have an "Officer's Quarters" next to it and armies built from the city will gain some sort of leadership/morale bonus, or alternatively the barracks could have a "Arcane Warfare Academy" built next to it and armies built from that structure could gain some sort of of resistance to magic. However the player would have to make a choice, as they couldn't build both next to the barracks.
Of course even this doesn't really require adjacency to be implemented, because upon building the "Officer's Quarters" the developers could design it so the "Arcane War Academy" is no longer able to be constructed. In doing so the adjacent requirement becomes moot.
2) The add-on structure could alternatively enhance several kinds of buildings, but you have to choose which by building it adjacent to the structure you wish to grant the advantage. For example, an "Arcane War Academy" could still be built next to the Barracks in order to give your armies a magic resistance boost, however it could alternatively be placed next to your schools in order to boost research, or spell points or whatever.
So I'm not saying there aren't ways of creating choice and strategy around the idea of adjacency, but in all the discussion surrounding the topic I rarely seem to hear the strategies and choices afforded by implementing adjacent structures enhancing existing structures.