I wonder why some vehicles have a fuel tap and some do not have to shut off the fuel supply and to know when the fuel level has reached a reserve level. I have seen this is some motorcycles but have not seen in a car here
Motorcycles have a fuel cock because their simple engines are gravity fed. The gas tank is on top, and fuel will naturally flow into the carburetors. While the engine is running, this is fine, but the carbs are not meant to be a long term valve to stop fuel flowing. When they work correctly, they should stop the fuel, but many things can happen that would allow them to leak a little. Having a fuel cock on the bottom of the tank is a manual, definite way of stopping the fuel flow.
Cars do not have a fuel tank on top of the engine, and without a fuel pump running, no fuel is going to come out of the tank. No valve is needed.
As far as a reserve tank goes, that's just in place because a basic motorcycle doesn't have a fuel gauge. You need to watch your milage or actually look into the tank to check the level. The reserve is there to get you to a gas station if you miscalculate and find yourself almost out of fuel. With a car, simply opening the fuel lid and looking to see how much gas is there was never really an option so a gas gauge has been a part of a car for a very long time.
Modern motorcycles use a vacuum actuated fuel cock that automatically opens when the engine is running. They will also have a fuel gauge and "almost empty" warning light instead of a reserve cock. I believe that even fuel injected motorcycles have a vacuum fuel cock because gravity is still going to pull fuel out of the tank if it can.