The centre of a roof has weight pulling it downwards. This pushes the edges of the roof sideways on the top of the wall. To prevent collapse, there needs to be an equal and opposite horizontal force acting inwards on the wall. This is provided by struts between the two walls. Originally this was a wooden beam. These were later replaced with metal struts which can be seen if you look carefully. The wooden beam was cut off I the middle and the ends decorated with angels!
