#1: It's not healthy for the dog, especially if it's a food that dogs really should not ever have. #2: It teaches the dog that they get to have table food and they'll beg for it at the table, which can be annoying, and/or expect to see it in their dish and start refusing their own dog food, which is much healthier for them. This is one of the "best" ways to end up with a severely overweight dog, which can then lead to health problems like hip dysplasia, arthritis, diabetes. It's so much easier to just not get started on that path.
It has nothing to do with the food. Rather, it's all about the dog's eating habits. A dog has a feeding bowl and the more you feed it from the table while you eat, the more it gets used to eating in that way. The dog might start to beg at the table during meals.
Dogs have different dietary needs than human. Dogs are primarily carnivorous, while humans are what I call obligate omnivores--for good health, we need a wide range of foods from different sources. Dogs evolved to need a much more restricted diet. And some human foods are downright toxic for dogs: some examples are chocolate (coffee & tea, like chocolate, contain theobromine, which raises a dog's heartbeat to dangerous levels), raisins, wheat, and corn can be bad for them. If you do a search online, you can get a list, but even that may not list everything. And, as others have mentioned, your dog's manners may suffer if he expects to eat from the table. For a list of these foods, go to: www.webmd.com