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!
Taken from
Caree.org
For me I had a dual pedal when I got my mountain bike, I had never used clipless pedals before but my bike had that as an option. I rode if for a day or two with regular shoes and could feel I had been off the bike to long. I bought some riding shoes with clips to see what the buzz was about. What a world of difference it makes to have your legs power the stroke coming and going. I noticed my quads burning when I was climbing and I knew I was getting the benefit of the upstroke too. I will not ride my bike without being clipped in, in fact I upgraded my pedals to be clipless only, no more dual pedal thing. Just remember to adjust your tension on the pedal so you can unclip when stopping or you might just fall over while being trapped to the pedal, I do it about once a month.