For professions or companies where in-person collaboration is still needed, bigger concentration of business helps focus infrastructure investment into areas where it is most impactful. For instance, it's easier to make a subway line go through an area with lots of offices than it would be to have multiple separate lines passing through satellite hubs.
This stuff is mostly not centrally planned. People are free to open up offices wherever zoning allows (which is generally a lot more places than the few ultra-concentrated areas like downtown Manhattan), yet they tend to concentrate. Perhaps highly concentrating workers and businesses creates the best conditions for free movement of workers between businesses, allowing the best companies to more easily find the best workers.
It's true that these aren't centrally planned, but good infrastructure provides a feedback loop. People want to work at an office that's easy to commute to, so offices open near good transit. The extra offices require more transit, so more investment goes to the same area. Repeat ad infinitum.
