The pedals on your bike really serve only one purpose - as a means to transfer power from you to your bicycle. For riding around the block, they don't need to be very fancy... just plastic blocks with grooves or teeth to plant your feet onto. But if you want to go on longer rides (anything over 10 miles), you'll benefit greatly from something better, because:
Without something holding your foot securely to the pedal, it would be easy to slip off the pedal and send your foot into the wheel. Not so likely to happen on a trip around the block, but on a longer ride, when you're tired.
There is a correct placement for the position of your foot over the pedal axle
A good pedal/shoe system has to be able to transfer all of the power from your leg to the pedals without trying to bend your foot over the top of the pedal, which causes both fatigue and pain
You shouldn't have to think about how your feet connect to the bicycle while you're riding. You should be concentrating on having fun!