Why computer used Binary Number?

Answer 1

The (transistor) switches in an electronic computer, have only two states, ON and OFF.

Binary has only two numbers 1 and 0.

Large numbers can be represented by digits (bytes), made up entirely of ones and zeros.

Answer 2

Many early computers used Decimal Numbers (ENIAC, UNIVAC I, IBM 650, IBM 702, IBM 705, IBM 7070, IBM 7080, IBM 1401, IBM 1620, etc.). However the circuitry to do this is more complex than the circuitry to use Binary Numbers. Also software can easily be written to convert between decimal & binary forms, removing the need for complex decimal circuitry that early designers often felt was required.

Russian designers built some computers that used Trinary Numbers. However circuits that reliably identify 3 states in one signal are harder to design than circuits that reliably identify 2 states in one signal.