It was used in Chinese south-pointing chariots. The gear was only announced to Europe by Aristotle in 400BC. Right after that various inventors started using the gears for new inventions like the clock and a good deal of water raising devices. It is unknown who invented the gear, but historians think that Ma Jun (creator of the Chinese chariot that used the gear) invented the gear.
What was it made for: The gear was made for a Chinese invention the south-pointing chariots created in the 27th century BC, the chariots work like compasses, but they use gears instead of magnets. Whichever turns the chariot makes it will always point south. The gear wasn’t used after that for any known invention until, 400BC when Aristotle retold the world about the gear.
How it was used: The gear was used in the 27th century BC for one invention, but the gear was mainly used from 400BC till 150AD. Inventors invented many things like the clock, the sun and planet gear, the antikythera system, the automatic doors and it was used for more recent inventions like the bicycle, the motorcycle, the chain drive and even the all-new virtual reality