The 120° hub enables the creation of hexagonal and angled stations. It has the same dimension as the standard hub. Instead of four ports at a 90°, it has 3 ports spaced at 120°. The larger spacing between ports means that it can fit all existing parts without modification.
The long hub provides a way to dock large modules side by side. Two standard hubs don't quite provide enough separation to dock small modules side by side. While possible it requires a slight overlap between the modules and docking must be done carefully via the docking auto-alignment glitch. The second module must be docked at an angle away from the 1st so that the connect before the sides collide. The process of auto alignment straightens it out, but doesn't do collision detection so the the two small modules end up docked but overlapping slightly. This also causes them to shoot off wildly or explode when they are undocked.
The long hub it the height (or width depending on how you look at it) of a large station module. This means that if you connect two long hubs together via port 1 (the port with the white dot), you can dock in two large station modules side by side without any overlap.
The long hub can also be used when a hub is too short, but adding in a standard station module would add too much length.