Why do computers use binary numbers instead of decimals?

Answer 1

Because computers store data physically, and the simplest way to store something is through a switch, either on or off. So the whole storage system on a computer is based on this, it's just a series of trillions of switches. Decimal numbers would require far more complex systems, and wouldn't allow for the amount of storage our computers have.
Binary is more direct and requires minimal translation. 1s and 0s represent transistors on or off, which is the most basic.
Binary is a set of instructions used to control the computer, and works from 1's and 0's, but the computer understands them as on or off signals. If the decimal system were used, there would need to be 10 different voltages, in which case there'd be more room for error with resistors etc., and therefore more room for corruption of data.

Furthermore:The computer may process enormous amounts of data. But, because humans don't want to think about so much data in detail, there's a limit to how much of it is going to be input by humans or output to humans (eg typed in, read from the screen, etc). So the computer is only ever going to spend a small amount of time converting between one base and another. So when working out which base is the best, you don't really need to worry about converting bases to keep humans happy.

As one can see computer or to be more precise hard-disk is nothing but a box containing magnetic tape. Now one can actually think why magnetic tape? Why not anything else? Because magnet has two properties attracting (unlike poles) and repelling (like poles).

Let me give 1 to attract (for e.g.) and 0 to repel. Now when an instruction executes, a reader on hard-disk (as in cassette or tape recorder) sends signal to hard-disk . And hard-disk reacts according to signal. Let us say signal elements were 101.and now on seeing 1 it will attract and mark on magnetic tape will be seen as per it. And for 0 it will repel and leave some another mark. This will continue till the whole input signals are executed.

Now one can tell for all numbers 0 to 9 will be converted into binary (0 & 1) and then only it will be executed. This is much simple for a computer as it has got to do only two actions for any number of inputs.