Computers use the binary number system, or base 2, because they have two electrical circuits. On and off are the two electrical circuits used in the operation of a computer system.
The binary number system uses the numbers zero and one to calculate. In the base 2 system, each number is worth twice the value of the binary digit to the left of it. This determines its name of base 2.
Another number system is that of base 10. The base 10 system employs digits or fingers. Each number to the left of a a digit in the base 10 system is worth 10 times the amount of the number on its right.