To keep their (concentrated, moist) sugars from oxidizing, and effectively becoming caramel-like or tar-like, and hence less appetizing to the animals that are going to be tricked into distributing the seeds. Or rather, appetizing for too short a time.

Multinationals put anti-oxidants into junk food to promote shelf-life. Nature puts anti-oxidants into fruit to promote branch-life.

Warning! I might just be making this up. Check more authoritative sources if this info matters to you.

Edit: It occurs to me that the essential oils (fragrances) are also subject to being oxidized into something less savory than the original. Maybe more at risk of oxidizing than the sugars.