“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 an additional benefit”should work in most contexts. It just substitutes “additional” for the possibly-less-formal “added”, and “benefit” for the definitely-less-formal “bonus”. You can also go in-between in formality by using “as an added benefit”.
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