When a car is switching gears, the engine is providing no forward power. And it’s more difficult to steer, brake or otherwise control the forward motion of the car as you change it from one gear ratio to another.
And yet, the only way to effectively switch gears is to do it while moving. To use your forward momentum to overcome the pause in the engine’s power.
If you keep trying to accelerate without switching gears, you’ll blow out your engine.
It’s a pretty useful metaphor all around.