The goal of a birth plan isn't for you and your partner to determine exactly how the birth of your child will occur - because labor involves so many variables, you can't predict exactly what will happen. A birth plan does, however, help you to realize what's most important to you in the birth of your baby.
While discussing about birth plan, you'll be learning, exploring, and understanding your labor and birthing options well before the birth of your child. Not only will this improve your communication with the people who will be helping during your delivery, it also means you won't have to explain your preferences right at the moment when you're least in the mood for conversation - during labor itself.
A birth plan isn't a binding agreement - it's just a guideline. Your doctor or health care provider may know, from having seen you throughout the pregnancy, what you do and don't want. Also, if you go into labor when there's an on-call doctor who you don't know well, a well thought-out birth plan can help you to better communicate your goals and wishes to the people helping you with the labor and delivery.