This is an rarely known shoemaker tool. Waist pliers are used for same purpose like the well known "Bulldog lasting pliers" or "Waist lasting clamps". In a German tool catalog from 1936 they are called the same "Gelenkzange". They are made to last the waist on 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 to stretch sole leather on a table or board.
If you have any questions or special needs, please ask.