Exarchs are powerful tools of the gods, and have additional uses outside the normal expenditure of power points. Instead of counting towards your influence on the world they allow a god to influence themselves and other gods.
Create, Mold, Shape, Nourish, Guide/Teach, Spawn, Imprison
Exarchs that generate PP towards keywords count against the number of PP producing artifacts a god may have at a time. Exarchs that generate a combat bonus count towards the total combat bonus you may recieve.
In addition to their mechanical bonuses, exarches provide something else as well. An exarch is typically an intelligent being, and is capable of roleplay as well as taking actions using your PP. Most valuable is that your exarch allows you to influence events when your god is not present, but the exarch is.