Adding the numbers is not the problem, the problem is what the numbers represent, where do they come from (if they come from only one place...) where and for what are they needed.

Quickly you have to consider different kinds of numeral systems, bases, decimal separators (which may be locale dependant), precision, units, the difference in adding time (or dates), or money in different currencies (with and without different kind of taxes), etc, etc, etc.

Sure most of the times you can abstract yourself from all this, use sensible defaults, whatever, but that complexity is still there and sooner or later it will get you, even if you try to hide it under the rug...