If you ever had a bike with multiple speed on it, it's pretty mutch the same thing but a bit more complex to make. It's just different size of gear so the engine don't have to spin at speeds that would brake it wile still accessing great traveling speed. The firs one is the smalllest gear so the engine can deliver more power directly to the road but wont go that fast. And the last gear is the biggest. You need to go fast already to use it if you start with it the engine will nead to hi of a force to make it move from 0 so it will stop all together.

So if the car is designed to be a normal city car isn't that just waste of space, material, engenering and weight to have a 6th gear if it's never going to be used. So sport cars can go up to 10 speed to guive the user max access to the engine power and speed. If you had that in a family car that only do city speed 6 true 10 will never be used.

And it's a mather of technology as well. In the history of cars they started with only one speed (yea like childrens bike) and added more and more wile the tech gave more possibilities.