"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".
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.
I think the word 'additionally' comes close to the meaning you're looking for, however it does not have quite the same positive connotations as 'as an added bonus', but in the example you give it would fit well as a replacement.