I agree with Kelly, They are not essential, but they pose a great opportunity for you and your partner to become aware of your preferences and writing it down can help you have a stress free labour. Particularly if you had something last time and you definitely don't want it again. I think it really helps DH's/Partners to know what your feelings are about certain things (pain relief) especially if the unlikely happens (Em. C/S), he can really act as your advocate as he already knows how you feel about certain things. Nothing is set in stone though so if you change your mind about something that's different to the plan it's really not going to matter.
I like to write mine and have DH read it so he can be aware of my preferences also really makes things visable and eased in my mind, before the big event.
Congratulations on you twins.