Explain why fractions need a common denominator to be added or subtracted?

The reason why is that a fraction is a different numbering system than whole numbers. The "whole number" system is based on units of 100. But a fraction is only a part of a whole number, so in order to add or subtract you must make both fractions the same type of unit just like whole numbers. You cannot add 1/6 and 2/24 unless you have the same units...which in this case is 24. So, multiplying the six by 4 will give you 4/24, and now you can add the numerators together because they are in the same "family".