Well, it wasn't exclusively... but the diagram says what it is, where as the turbo lift diagram seems intent on showing the turbo lifts (though by the time the series went into production Jefferies (and the producers) wanted it more extensive to get people from place to place better. The other lines aren't denoted, and could be things like corridors or the like.
As for why use the compartment diagram at all?
It provides a very nice partitioning of massive decks. Other wise you start with nothing and the question pops up later as to how the compartments come into play. But mainly it was a better starting point than big blank circular decks.
The top view of the secondary hull in the turbo lift diagram is more helpful though, and if you look at the floor plans of the secondary hull that I've done you might see where I sort of followed that partitioning rather than the other one (which had bulkheads down the center of the hull).
But again, it is not exclusively used... it is used in concert with a number of other references (Jefferies deck heights, the actual set dimensions, the physical shape of the model, etc.). I'd have reached different conclusions if I ignored any of them or weighted them differently (and other people have obviously reached different conclusions), and my conclusions might change again if I find more data.
All I'm doing is distilling and refining all the data as I see it... which is why I'm also attempting to make sure people have the data so they can distill and refine
it in their own way. I'm not saying (and never have said) that my interpretation is the only one or even the best one.
All this really is is my best efforts.