Egyptians believed that cats were magical creatures that could bring good luck. On the other hand, animals like dogs were valued by Egyptians because they were good at hunting and protecting them. In general, animals were valued by different reasons such as the ones stated here. As the question says, there were many gods who could adopt animal forms, but just Bastet could become a cat.
Portraits were important in early history because if we had found those paintings and pictures we can determine what there life back then was and what materials they had in the past then what higher materials we have now.
