Hold your dog's lip back and look at the top of its molars. You'll see a dark, uneven line of stuff where the teeth emerge from the gums. While you're there, look at the bottom edge of the gums. Chances are good the edges will be redder than the gums themselves. That redness is inflammation caused by the plaque.
Some dogs get plaque worse than others.
Diet is important, but probably not as important as genetic tendency. Since it is a home to bacteria, it is a chronic low-grade infection in the dogs body. Fighting that infection over the years will take a toll on the dog's liver, kidneys, heart and lungs. Plaque can cause serious gum problems, far more than the inflammation you can see. Plaque can create serious problems in the bones supporting the teeth.