When you integrate, why do you need to add a +C on the end?

Integration can be used to undo differentiation. So if you had an equation that was y=5x+2 and differentiated to get dy/dx=5. When you integrate it you would only get y=5x, so the +2 is lost, to make sure you take this number into consideration you should always put a +C on the end of the equation. This means that it would be y=5x+C.