These tacks are designed with a large flat head to give maximum contact with canvas or fabric. The large flat head, as well as a longer shaft, provide greater grip to help secure your ground fabric to your stretcher bars. They are especially useful when using custom bars like Evertites, as they are less likely to be rolled out by tightening.
It is best to remove the tacks by slipping something under the fabric/canvas and pulling up. I use an old butter knife. If a tack should bend (and being long, they sometimes do) simply use a small needle nose pliers to straighten it. It can be reused – and reused.