Since the advent of the LCD screen, one of the traditional commandments has always been never to touch the surface. Contact with the screen can distort images, ruin pixels and wear out a pristine monitor quickly. This still applies to standard desktop and laptop monitors, so if you like to take the hands-on approach in your computing affairs, stop reading and get a screen protector pronto.
As we proceed into the age of smartphones and touchpads, those of us used to the “no touching” rule find ourselves in a foreign, backwards world where touching the screen is not only acceptable, it appears to be necessary to use the device. All the new toys use capacitive screens, which require something conductive in order to work... such as a human finger. It's madness, I tell you.
Capacitive screens are protected from the aforementioned dangers, of course, but that doesn't mean it's always pleasant to use fingers to touch them. Screen smudging is an issue, fingers may not always be clean and jobs may demand a stylus for better accuracy. Most PDA styli don't work since they aren't conductive. It's a problem that had gotten so bad that folks in South Korea started using sausages once they discovered, through what had to be some fantastic trial & error experimentation, that they somehow worked. Thankfully, Targus has a conductive stylus on the market now.
Making a conductive stylus is more complex, so they aren't cheap and only recommended for those who either need the accuracy of a stylus or dislike sausage.
If you're only worried about screen smudging or just can't stand the thought of making direct contact with an LCD screen, try 3M's Screen Protector Film. It can be cut to the size of your phone and reduces smudging while keeping the device fully functional. You can feel better touching this with your fingers or, if you still prefer, your favorite encased meat.