[D] what do the two zero convolution layers in controlnet do?

in the paper "Adding Conditional Control to Text-to-Image Diffusion Models" (2302.05543) they add 2 zero convolution layers. the paper emphasise that in the first forward pass the output is the same (since the layer is all zero) and after the first backpropagation, the layer is not all zero anymore - this is ok, just math.

however it does not explain why was is added? or what does it do? how came they initialize it with zero (when all other papers …

backpropagation control controlnet convolution diffusion diffusion models image image diffusion layer machinelearning paper text text-to-image