Corner point constraint in Stress analysis for aircraft fuselage panel

In Stress analysis for aircraft fuselage panel.

One particular BC is going over my head.

panel_origin = (-0.5, -0.9)
# corner point : bottom left corner (why this is needed??)
    panel_corner = PointwiseBoundaryConstraint(
        nodes=nodes,
        geometry=geo,
        outvar={"u": 0.0}, # why ?? I don't know??
        batch_size=cfg.batch_size.panel_corner,
        criteria=Eq(x, panel_origin[0]) # this is bottom left corner. Why??
        & (y > panel_origin[1])
        & (y < panel_origin[1] + 1e-3),
        param_ranges=param_ranges,  # parametric hoop stress
    )
    domain.add_constraint(panel_corner, "panel_corner")

Why do we need this BC and only at the bottom left corner?