Strictly- you can only add values that are the same units. 2 apples + 2 apples = 4 apples. 2 apples + 2 bananas = ... 2 apples and 2 bananas. Multiplication however, you can multiply anything together and a new unit. 2 apples x 2 bananas = 4 apple-bananas. In short - multiplication is "legal" - addition is conditional on units.
