The suggestion is that it should be cheaper to build a Maker's Quarter beside another Maker's Quarter, rather than building it beside a Farm or Market Quarter.  The first quarter of any kind, plausibly, would make it very easy to expand upon adjacently; and more costly to recreate again in a disparate location.

In the real-world, it's usually cheaper to expand an existing industrial center, or market center, than creating a new one, from scratch, somewhere else.

I believe the game Old World does this, offering a 20% discount when building "same adjacent buildings."