Your phone is an incredibly personal device. More than likely it has some very personal and private info saved on it. You wouldn't want a friend, coworker or kid happening up on Emails, Text messages, videos, and photos that are for your eyes only. Lockscreens protect your device while it is laying around from those who would be nosey enough to snoop around and take a look while you are away.
There are those times when someone ask to use your phone to place a call, or maybe you hand them your phone to read an article you have pulled up. Your intention is for them to only do the task that you have agreed for them to do, but you can't stop them from opening up other apps and getting all up in your business. CyanogenMod has a nice little privacy feature built in called Folder locking. You can place all your private apps in a folder. When you open that folder you will see a padlock, clicking on the padlock will prompt you to make a swipe pattern, after setting up the pattern the folder will appear as a lock icon rather than a folder full of apps. You can have many app folders, and as long as your apps are in a locked folder you won't be able to find them in the app drawer. This is just one more reason to use a Rom like Cyanogen!