An abacus was used anciently to count using beads.

The same as it has been used for thousands of years: doing calculations.

An expert abacus operator can visualise numbers and do calculations (usually) faster than someone with an electronic calculator - using an abacus visualised in their head, a skilled operator can often do calculations faster than with a physical abacus.

Abaci have the advantage that the batteries don't wear out (as there aren't any), which makes their use in places with limited supply of power preferable.

Yes, abacuses (or is it abaci?) were used to calculate simple maths problems.

The answer is "you can count on me."

But of course, you do not "count" on a calculator: you calculate. You can count on your fingers, or on an abacus, or a handheld tally counter.

156... without using a calculator !