I have always had insurance either through my parents, through school or through work so I never went to PP. Several of my friends when I was in my teens and twenties used PP for their annual exams and birth control when they did not have insurance. I still have a couple friends who go there - they are on Medicaid.
PP usually does not perform mammograms, but they do the a physical breast exam as a pre-screen and help a woman who needs a mammogram get one for free or at a reduced price. This physical breast exam is part of an annual OB.GYN visit required for BC prescription.
The cancer screening that planned parenthood does the most is cervical cancer. Pap smears are cervical cancer screening. You cannot get a prescription for oral contraception without an annual pap smear. If you do not have insurance a pap smear can cost you several hundred dollars. That is the barrier for most low income women to getting the pill.
The majority of what PP does is provide the annual gynecological exam, including a physical exam of the breasts, ovaries and cervix along with a pap smear, that is a prerequisite for oral contraception.
I am realizing more and more than many men have no idea about these things. A pap smear (which is collection of cervical tissue scraped using a stiff brush that is sent off to a lab for observation under a microscope) is required to get a script for birth control pills. These tests catch cervical cancer early while it is still treatable.
Long term use of oral contraceptives can increase the risk of breast, ovarian, and cervical cancers. So the regular screening for these cancers is part of prescription process.
PP provides these services to low income women, people without insurance, or people on Medicaid. That is the bulk of what they do - give low income people access to birth control with the health care procedures required to get it.