Pliers can be used to grip parts, more easily extract pressure-held parts (like fuses), and to loosen and tighten nuts, bolts and other connectors. They are particularly useful when dealing with a tight connector, such those on the speedometer cables, or a part that is hard to reach or get a good grip on with your fingers. For most nuts and bolts, a fixed wrench or socket wrench should be used instead. Two sets of pliers can be used to bend thin pieces of metal.
There are many different kinds of pliers. See the Wikipedia pliers article for a number of various plier types. Slip-joint pliers can be particularly useful for grasping the aforementioned speedometer cable connectors. Needle nose pliers are useful for grasping small parts. Pocket multitools such as those from Leatherman often have built-in pliers, screwdrivers and other tools and can prove handy when a failure occurs on the road.