If there's one thing you take away from reading this poem, it's that Chicago is a tough place for tough people. It's a rapidly industrializing city, and we actually see it being built, wrecked, and rebuilt. The poem is a little obsessed with the strength and power of the city, and, by extension, the strength and power of the Chicagoans themselves. And this was before Al Capone showed up on the scene! In fact, the poem's personification basically portrays the city itself as a strapping young laborer—a strapping young laborer who is building the city itself. "Chicago" is not just about the powerful position of the city, but about the brute strength of the people who built it.
The city of Chicago isn't strong; its inhabitants are strong. (It must be all that tool making and wheat stacking they do.) The city of Chicago is powerful because its inhabitants are so violent. (It must be all that hog butchering they do.)