The Community site makes it easy for you to control who can and can't send you messages and comments or view your photos. Profile pages are locked away from non-members but they are viewable to registered members on the site (and this is preferable if you plan to build a healthy base of friends and contacts)...
But some members may prefer to keep their profiles and pictures locked away from people they havn't vetted in some way. By making your profile private only friends can view it - and you get to choose your friends! So if someone makes a friend request and you're worried about privacy, you can take a look at their profile first and find out a little more about them before letting them into your network.
All you have to do is go to "My Page" and check the privacy question in the Hello box.
Please note: Other members will still be able to view your pictures and blog posts unless you choose otherwise. You can change who can see your pictures from My Settings > Privacy.