Let's say you have a map of the streets in an area. Could you guess, from looking at that map, which intersections the locals consider "most important"?
You could probably make a decent first guess by assuming that the importance of an intersection goes up with the number of roads at the intersection.
Why? Imagine you are randomly walking around a town. Each time you get to an intersection you choose uniformly at random from each of the possible ways you could go. (For the sake of simplicity, I'll assume that this includes going back the way you came.) It's a well-known result that this sort of random walk on a graph leads to a certain stationary distribution; that is, if a bunch of people walk around randomly you'll get to a point where the number of people at any given intersection is roughly constant. And that result is that number of people at any intersection is proportional to the number of roads coming into it. (I'll count the two directions of a road coming into the same intersection as two distinct roads.)
This has implications if, say, you want to start a business. If you're located in the middle of a block, there are two roads coming in. If you're at a "T"-shaped intersection, there are three. A normal grid intersection, four. An intersection like 48th and Baltimore or 23rd and South, five -- both of these have two streets that go through and a third that starts there. (Most of the intersections on Philadelphia's Baltimore Avenue are five-pointed, because the street grid has a sort of discontinuity there; this may explain why it's sort of the main street of its part of the city.) An intersection like Broad and McKean, which has three streets going through it, six. You want to be where there are more roads, so that more people will walk by. In Washington they have intersections like Dupont Circle where five roads go through, for a total of ten "arms". (The counting gets a bit tricky, because some of the roads don't actually go "through".) In Paris there's an intersection with eleven arms, fittingly called L'Etoile. (Much more than that seems impractical.)
Of course, this neglects a huge number of facts. Not all roads are equal. For example, I'm ascribing Baltimore Avenue's primacy in its part of Philadelphia to the fact that it's got five-pointed intersections; but probably more important is that, historically, it was the road that went to Baltimore. (Hence the name.) Also, a trolley runs down Baltimore Avenue and has for over one hundred years; but why is it there, and not somewhere else? Because people already were living or working on or near that street. And they were doing that because other people were. Once a slight imbalance is created -- say by the random-walk model I alluded to above -- people are naturally going to gravitate, even if it's very slightly, towards the place where people already are. The roads that lead towards the important nodes will become important in their own right. And street networks aren't static -- they can change. (River networks can't, though -- and a lot of cities are located where two rivers come together to form a third, the most famous U. S. example probably being Pittsburgh.) But the initial imbalance has to come from somewhere, and this is as good a place as any, especially in places where the roads mostly form a grid and hence their arrangement is determined well in advance of their actually being built.
Incidentally, this is my theory on why Boston and Philadelphia -- two cities which are of similar size and population density, at least if you consider them as the center of their respective metropolitan areas, and the two cities I am most familiar with -- differ in their transit system. Boston is able to have subways because for the most part it's obvious where the subway stops should be. Radical Cartography has a map of Boston as a series of squares; the original transit system was basically built around people getting from one square to the next. Philadelphia would have trouble supporting a subway system more extensive than its current two lines, because it's hard to point to a place that a huge number of people go which isn't served by the existing system; the population is more diffuse, because for the most part Philadelphia's streets form a grid and no node is any different from any other. If I wanted to I could draw, say, six or eight subway lines that I'd like to exist in Philadelphia -- but none of them seem essential to me, at least at the price that it costs to build a subway. (Some Philadelphians will point out that there's a long-standing plan to put a subway down Roosevelt Boulevard; to them, I will say that I have very rarely had any reason to be in Northeast Philly, so I forget about the Boulevard. I'm sorry.) Manhattan has subways -- even though it's for the most part a grid -- because it's so dense. (But I think I heard somewhere that Times Square is the busiest subway stop -- and it's under Broadway, which cuts through the grid diagonally.)