The blacksmith leg vice aka post vice is an important blacksmiths tool. Like any vice a leg vice is designed to hold things. It differs from your regular table mounted vice in that a leg vice is designed to hold up to the pounding that occurs with standard blacksmith methods.
A typical leg vice uses are forged not cast iron or ductile iron like most vices. The most noticable difference is the leg which provides support from the floor or from a sunken post. The last difference is the leg vice is hinged which is much more durable and can hold up to pounding.
When the leg vice does fail it is usually the pin of the hinge that fails. This part is easily replaced.