One thing that you will need is a Caribbean map if you looking forward to visiting the Caribbean.
If you only plan on visiting St Lucia, then a map is no a must, but it a good idea to have one if you plan to island hop in the Caribbean.
An other reason maybe because you want to familiarize your self with the islands in the Caribbean and know exactly where in the Caribbean you can find St Lucia.
You may choose to buy one, although it is not necessary. Some websites allow you to print one off their site so that might be sufficient for you, although it may not be detailed enough.
I personally like using Google maps when ever I need to take a quick look at a map for any where in the world. But if you need a more detailed map that you can carry around, then you may opt to buying one.
For your convenience and edification, I have placed a map of the Caribbean here.