What form of CP do you have?
I'd imagine it is just so that you are not struggling trying to deliver a large baby. Some women with a small pelvis have to do the same thing.
Or so that there is not any more stress put on your body than can be avoided, especially if your CP is affecting muscle tone (they can't tell how it will affect your ability to push till the time comes, think of children with CP and toilet training, they don't know how internal muscles are affected) it is tough on 'normal' bodies without the added strain that comes from the CP (I'm thinking of ligament strain from supporting a full term baby bump).
Have a chat with your mw or consultant to find out exactly why they are planning this, especially if you are worried.