Many dog owners have very well behaved dogs, and "reward" them by walking them off the leash.
This is not the best of practices for the dog's health and safety.
Even the best trained dogs can have moments of urgency. Depending on the dog, a squirrel, cat, or bird could make Lassie take off after them. The danger is of the dogs running into the street, and possibly getting run over, or of them running away from you, the owner, and possibly getting lost. Also, there are a multitude if chemicals out on the street, from engine oil to Anti freeze, that your pup may lick up. A dog's tongue gives it a lot of information about other dogs, or the world around them, so it is natuiral for a them to want to lick things, just to check them out. A tongue full of anti freeze can cause serious injury, or even death. A dog not on a leash can go to a foreign puddle and lap it up before you get to it.
If this is a smaller dog, there is always the danger that a larger dog will see it and try to attack it, chasing it away, and leaving you helpless to do anything to protect your beloved canine.
Another thing we usually don't think about when it comes to the leash is the connection it gives us to our dogs. They, after all, look to us for guidance and leadership. Your energy dictates to your dog how it should behave more then your words ever could, and during the walk, that leash acts like a conduit, allowing your energy to flow down to your pup and let it know what you are thinking.
A bit metaphysical, I know, but you get my point. It is your link to you pooch.
So, when going out for a walk, remember, it is in your dog's best interest to keep that leash on. Lead it, guide it, and most of all, keep it safe from harm.