Why do we need a common denominator

Finding a common denominator makes it possible to add two fractions because it allows us to write each fraction as a multiple of a common (usually smaller) fraction. Subtracting fractions works the same way; find a common denominator so that the fractions involved are in the same terms.

[ Source: www.mathmistakes.info/facts/AlgebraFacts/learn/fr/add.html ]

If u do not have a comon den. that it will be confusing and u will not come up with the right answer