Public trees facilitate your research and those of other members:
By making your tree public, you increase your chances of being contacted by people doing the same research as you. They certainly have information that may interest you. Your shared work allows members of the site to advance faster in their research. At any time, you can choose to make your tree private.

Your information remains protected :
Information about people that are still alive on your tree are kept confidential and not accessible by research. Only you can modify your tree, other users only having access rights.