The 1.1.641 patch (Rush Hour) adds the mode, allowing you to take control of vehicles. While this sounds like a gimmick, it serves a purpose: you can drive a bus route to see where your transit system is failing, or pilot a helicopter to fight fires. It bridges the gap between the mayor and the city.
While Maxis never released exhaustive patch notes, community testing and reverse engineering have identified the main changes from 1.1.640 → 1.1.641: simcity 4 1.1.641
Because 1.1.641 is a legacy application, further tweaks are often needed for modern hardware: Resolution - SimCity | Fandom simcity 4 1.1.641