You hit the nail on the head, here. Even if you're in a committed relationship, there's still a need, as a woman, to protect yourselves at all costs from pregnancy if you aren't looking to have a child. That's why many women in relationships will use secondary birth control. (Condoms + the pill, IUD + spermacide, etc.)

Female birth control puts the power of reproduction back in the hands of the woman, which is a comfort for most women. Relying on your partner to be the one safeguarding your body is a tough choice, but is a wonderful choice for, as you said, couples.