The purpose is to stop hoisting down the cradle when the bottom of the cradle experiences an obstruction.
This may occur due to the following reasons
An open window is in the line of the descending cradle.
A protruding obstruction from the façade below the cradle.
When the cradle reaches the ground level.
When the cradle rests on the roof floor during parking, hoisting further down by over-ridding the trip bar will lose the tension in the wire ropes which would hamper the set routing of the wire rope through the sheaves.