Your gearbox helps to increase the turning speed and power (torque) of your engine, sending it to the wheels of your car.
The speed and torque increase are useful for when the road speed varies, or when your engine requires more power such as, when you’re climbing a hill, towing, or even just taking off from the traffic lights.
Inside a manual gearbox you will find; gear oil, detailed gears, bearings, and linkages that all work in perfect sync when treated and maintained correctly.
Inside an automatic gearbox you will find all of these parts, as well as a pump, a torque converter, multiple sensors, valves, and much, much more. Essentially this means your manual or automatic gearbox is a very complex beast indeed.