Why does a computer use Binary number System?

Binary is the simplest way to represent information. Binary has two states, high and low. It's pretty easy to distinguish between two states. If you tried something more complex, like a trinary (low, middle, and high) you would have two boundaries to worry about, which could be hard to distinguish between. For example, a binary system could define 1 to be (near) 5V and 0 to be (near) 0V. You don't have to distinguish between 2.45 V and 2.55 V, just close to zero or close to 5. Or it could be magentized Left-to-Right or Up-to-down. You don't have to worry about is it 40 degrees or 50 degrees off.