"Go where the money is, and go there often."- Willie Sutton
The geographic expression of economies of scale, where various bits of related economic activity tend to clump together to minimize transportation costs or maximize some benefit.
This is the reason
- your grandparents started going to supermarkets rather than little mom-and-pop grocery stores with produce out on the sidewalk, or why you might go to the mall or that big-box store out on the bypass rather than downtown.
- automobile dealerships are found in clusters, since propsective car buyers are apt to go somewhere they can shop around first.
- a large factory's suppliers are often located on the road leading up to the main gate.
- so many "Beltway bandits" troll around Washington, DC.
- crocodiles wait at particular African river crossings during wildebeest migration season.
- boom towns form near the latest gold or silver strike, and turn into ghost towns when the mine is played out.
- diamond cutting happens in a small number of large urban centers far from diamond mines (high training costs and a small talent pool).
- fish form schools, birds form flocks, seals all gather on the same beach to mate, etc.
The effect is more pronounced as transportation costs grow relative to the overall cost of the activity. An industry whose transportation costs are negligible compared to other costs is known as a "footloose industry", but even then, economizes of agglomeration can kick in because other economic benefits are maximized by being in a certain place.