Why do you switch gears in a car?

If you,ve ever ridden a bike with gears where you are effectively the engine, or power source, then you will know.

It's hard to pedal uphill in a high gear and tiring to pedal very fast downhill or on the flat in a low gear. Using the gears properly uses the power source, ie engine, most effectively and prevents the engine getting more 'tired' than is necessary, or in other Words, damaged, worn out and needing costly repairs.