Mandelic Acid has antibacterial properties, regulates sebum production to help prevent the recurrence of outbreaks, clears pores, exfoliates, and increases cell turnover. For issues of hyperpigmentation and melasma, mandelic acid breaks up discoloration caused by such things as sun damage, post-inflammatory hyperpigmentation (PIH), and hormones. 
What makes this serum so special is that every other ingredient amplifies or compliments Mandelic Acid's properties. Azelaic Acid, Niacinamide, and Lactic Acid combine to reduce inflammation, exfoliate dead skin, increase cell turnover, strengthen  collagen to reduce the appearance of fine lines, and improve skin texture and appearance.