By default if you have chosen an alert style to view your SMS and iMessages the first line of your message will show up on your lock screen
or any screen while using your iPhone. There is an option to increase your privacy on upon receiving these messages allowing
only the senders name to appear.
Depending on where you are you may find yourself not wanting the first line of your iMessage or SMS viewable to those around you. Having the option to decide a bit of privacy may be helpful to some iPhone owners.