A whole variety of aliments...different herbs and aromas are used to treat different conditions. It is almost a science if studied correctly.
For instance, lavender oil can be used to destress and relax and calm.
Lemon oil is a natural antidepressent
Tea tree oil is a natural antiseptic

Aromatherapy practitioners explain that it is beneficial for many things. Some of the benefits and uses are: Relaxation and stress relief (and personally, I would say this is one of its most widespread uses since it is extremely effective); Mood enhancement for a general feeling of well-being and anti-depressant effects; Boost the immune system, and other body systems such as respiratory and circulation; Relax muscles; Improve sleep; Energize; Relieve headaches and congestion; Relieve menstrual cramps. I have also read claims that aromatherapy can improve skin tone and aid in healing skin conditions, balance hormones and reduce inflammation.