Used for breaking glass along the score when it is too small for fingers or running pliers. The difference between these and normal pliers is that that the jaws only meet at the tip. This prevents the damage caused to the glass when the jaws of normal pliers close on the delicate edges.
The shape of the jaws of these is also designed for grozing, which is the process of nibbling the edge of the glass to shape. Also useful for heaps of other jobs, these are pretty much indispensable for glass cutting.