"As an added bonus" seems quite casual. Is there a word or phrase that expresses it that is suitable in a 'serious' essay? For example, "... Thus, equations give us scientific knowledge. As an added bonus, equations are easier to use than their alternatives."
as a bonus would suffice. Bonus already means 'something extra', and is derived from the Latin bonus ('good').
as an added bonus is informal since it has a redundancy.