OK, so you used one form of birth control that either you or your partner purchased.
Maybe you'll get into a long term relationship or a marriage where a condom is no longer the preferred method of birth control, but you'd still like to have sex without pregnancy risk every time.
So you'd like other options to be available.
Most insurance plans have female contraception choices included.
Having access to "free" contraception helps you.
Now let's say your sister doesn't bother with birth control, gets pregnant and the father runs out.
In that case, your whole family deals with an unprepared pregnancy and years of raising a child that your sister can't afford.
Not you problem but it becomes your problem.
Or how about the woman you don't know at all that gets pregnant and uses your tax money to get welfare to raise the child?
Free birth control for her could really have saved you money there.