Locking the keypad is important because it will prevent accidental keypress. Accidental keypresses can make a phone call, delete a contact, and many other things depending on your shortcut settings. An automatic keylock feature should have been included on all phones, unfortunately, this was not the case with Nokia phones.
The owner of sliding phones should not need this application, but all others should get one. This application was written by Petteri Muilu, and can be downloaded from:
http://fuse.stc.cx/autolock/.