Oral contraceptive pills decrease the incidence of all of the following conditions except:
Friends, it is absolutely essential to mug up the benefits, side effects and contraindications of OCP's.
Many questions are framed from these topics.
Here, I am repeating the list of Non-contraceptive benefits of OCP'S: 
Cancers/cysts

Uterine cancer
Ovarian cancers
Fibroid uterus (Progesterone only pills)
Ovarian cysts
Benign breast diseases
Benign disease of the genital tract
Endometriosis (if used continuously)
PID (here Salpingitis)
Ectopic pregnancy (as it decreases the incidence of PID)
They decrease ovulation thus, are helpful in
Dysmenorrhea, premenstrual tension and Mittelschmerz syndrome.
By decreasing blood loss they are helpful in menorrhagia and polymenorrhea.
Acne and hirsutism (especially those containing desogestrel)
OCP's are also beneficial in:                                                                                                                       
DUB
Hormone therapy for hypothalamic amenorrhea
Prevention of menstrual porphyria.
Salpingitis
Hepatic adenoma
Ovary CA
Fibroadenosis