Abacus is an instrument used in performing arithmetic calculations. It's a mechanical aid used for counting; it is not a calculator in the sense we use the word today. The person operating the abacus performs calculations in their head and uses the abacus to keep track of the sums, the carrys, etc.
The device evolved from a simple need to count numbers. Merchants trading goods not only needed a way to count goods bought and sold, but also to quickly calculate the cost of those goods. Until numbers were invented, these counting devices were used to make everyday calculations.
