Why is estrogen combined with progestogen in oral contraceptives?

Minimize spotting: Break-through-bleeding (btb) is common with the progestogen (p) only oral contraceptive pills (ocp). Although btb can still occur with the combined estrogen (e) and p ocp, it is much less frequent, versus p alone. In ocp with high levels of e, such as ethinyl Estradiol 50 mcg, ovarian cysts and ovulation also occur less frequently. Way to go gregory pincus, et al.