Because that is what it is: It implements the identity deformation for three-dimensional continua.
The name 'wriggers-l-shape' refers to a simulation that appeared in
O. Sander, P. Neff, and M. Bîrsan: Numerical Treatment of a Geometrically Nonlinear Planar Cosserat Shell Model, (2016)
It belongs to the dune-gfe module, and only got into dune-elasticity by accident.