Why are RNNs used in some computer vision problems?
When I think about RNNs applied to Computer Vision, two main research areas of Deep Learning come up to my mind:

Image Captioning: Neural Networks trained to produce descriptions of images. In that case, you have a Conv ecoder that processed pixel data, and an RNN decoder that produces a description.

Video processing (I don't have a better term). Anything regarding video data requires a mixture of CNNs and RNNs. In general: you process each frame with Conv layers, and the sequence of processed image frames is then fed into some RNN layer that performs the final task, that could be classification, segmentation, self driving cars, ... you name it.