It does NOT decrease cervical cancer. It DOES decrease endometrial and ovarian cancer though for the latter not as much as ring/patch/pill. Endometrial cancer by stopping the monthly ebb and flow of the endometrial lining. Less growth/shedding, less risk of cancer. Ovarian cancer b/c it does block ovulation but not as 100% as the ring/patch/pill. again, less frequent damage to the ovary (ovulation) less risk of CA.
b/c people don’t use them correctly (re-use! or old condom, or put it on the wrong side, flip it [but the wrong side already has pre-ejaculate which can have s14% rate of failure with condoms, why?perm] and possibly breakage.