The long and the short of it is so that we don't spread diseases... Believe it or not, many people don't get their dogs the shots they need and fewer get them dewormed. If you're one of those people, please be extra careful with your dog's feces. Just take a few seconds to pick it up. You never know when you be causing someone else to have a sick dog and a huge vet bill.
For those of us who do get our dog's shots and dewormings, pick it up anyway. If your dog isn't on a monthly preventative, they could still contract parasites....Therefore spreading them to others. I know some of these medications can be expensive, but they are still cheaper than a dying dog's vet bill. And not to mention, if the infestation is bad enough with certain parasites... you just might loose your best friend.
For more information on parasites, the ways to contract them, and prevention, contact your local vet.