I think they believe it works, if that’s what you mean. There are no prohibitions in Buddhism, so it is allowed to use birth control, but it is preventing a being from being reborn, so it might only prolong your stay in Samsara – the cycle of rebirth. The person who might have been conceived and born had you not used birth control has karmic ties to you. Preventing that person’s rebirth could be a lost opportunity for all of you to clear negative karma. On the other hand, preventing a pregnancy could also prevent a lot of suffering in this life. Only an enlightened being can really know if allowing or preventing the pregnancy will avert the most suffering. Being mindful and honest with yourself, and cultivating compassion for yourself, your partner and anyone else involved can guide you in decisions about birth control.