They don't fill the hole, but they are made of soft brass so they shape to compensate for any irregularities in the head of the spoke or the chamfering of the spoke hole, which cause the spoke to crank more and break earlier. The spoke doesn't have to fit tightly in the hole, just sit squarely in the chamfered hole without torquing around. The problem with spoke head washers in many hubs is that the hub is already thicker than the elbow of the spoke and DT a couple years ago reduced the elbow length a bit. I generally like to use them, partly because if they prevent an irregularity on a spoke from digging into the spoke hole, they reduce the likelihood of breaking a hub flange.