The current way of distributing vehicles is somewhat inefficient. You can assign vehicles to depots/parking lots or to a stand. While assigning vehicles to stands is a very fool-proof system, it’s also expensive since your vehicles sit around doing nothing most of the time, while they could have been useful elsewhere. If you don’t want to go with this solution, you have to use depots. Having just 1 depot makes travel times very long, resulting in unwanted delays. So why not get more depots you might ask? In the current system, vehicles are randomly selected to go to a stand requesting a service. This is where the problem lies: the randomly selected vehicle can be in a depot at the other side of the map, while there is a depot closer to the stand that requested the service. This is why it would be useful to assign depots to stands, the same way you can assign baggage bays to stands.
To make things more clear, see the image below. As you can see I have two depots, one in the red square and one in the blue square. Vehicles from the blue square are sometimes requested to go to the red square, even thought the depot in the red square has vehicles inside. This problem would be solved if I could assign the blue depot to the stands in the blue square.