Why does computer consist of ‘on’ and ‘off’ (1 and 0) and not just ‘on’ (1)?

Answer

A computer is a device that processes data: numbers, texts, and so on. The smallest unit of data is a bit, represented by 0 or 1. Why was this system chosen? Simple, because a computer is made of components, which are all tiny little switches. Each switch can be “on” or “off” (1 or 0). From these two (binary) states you can represent any number or letter: the number 6 becomes 0110 and the letter A becomes 100 0001 with the ASCII encoding.

Why not just work with “on” (1)? Well, that wouldn’t be enough to encode information. You need at least two states for this.