In fact, firms in almost every industry tend to be clustered. Suppose you threw darts at random on a map of the United States. You’d find the holes left by the darts to be more or less evenly distributed across the map. But the real map of any given industry looks nothing like that; it looks more as if someone had thrown all the darts in the same place. This is probably in part because of reputation; buyers may be suspicious of a software firm in the middle of the cornfields. It would also be hard to recruit workers if every time you needed a new employee you had to persuade someone to move across the country, rather than just poach one from your neighbor. There are also regulatory reasons: zoning laws often try to concentrate dirty industries in one place and restaurants and bars in another. Finally, people in the same industry often have similar preferences (computer engineers like coffee, financiers show off with expensive bottles of wine). Concentration makes it easier to provide the amenities they like.
Q: Why did the author use "needed" (past tense)? Is it sequence of tense or semantic aspect?