This is an rarely known shoemaker tool, that I have nowhere seen in use. Waist pliers are used for the same purpose like the well known "Bulldog lasting pliers". In a German tool catalog from 1936 they are called the same "Gelenkzange". They are made for heavy boots with strong upper leather. With the waist pliers it is possible to load more strength on the upper leather than with the bulldog pliers.
See also the illustration in the tool catalog on last picture.
Alternatively, the pliers can be used as crimping pliers or to stretch sole leather on a table or board.
If you have any questions or special needs, please ask.