The shaman contacts his/her power animal for number of reasons. It is a guide, a teacher, a helper, and sometimes a savior.
Any animal can be a power animal. However, some don't consider domesticated animals as power animals. Insects and invertebrate are also mostly not considered power animals. Insects are seen as connected with medicinal plants, and their strengths lies right in there, not so much in helping the shaman otherwise.
While the shamans have a very close relationship with their power animal(s), every person on the planet is considered as having one. In many tribal communities, the shaman journeys to find a newborn's power animal, and let's the baby's parents know what it is. They then help the child build a relationship with its power animal.
From Christian's own experience, retrieving his prematurely born son's power animal a few days after his birth strengthened him enormously.
Shamans dance their power animals every now and then, giving this spiritual entity their bodies to experience the material aspects of life. This is to strengthen the bond between the shaman and the power animal, and as a thank you to the power animal for all its help.
Power animals reflect people's character traits. For example a person who likes to laze around on one side, but also bounce with full force and utter preciseness on their "prey" (a project etc) could have a tiger as their power animal.