why do most algorithm only take number between -1 and 1 as input or output?

[ichabod801]

Well, numbers from -1 to 1 have certain properties that can be useful. For example, if you take any two numbers in the range -1 to 1 and multiply them by each other, the result will be between -1 and 1. That is not true for the range -2 to 2. If you're doing an iterative process this can be useful.