I think they go hand in hand. Clients need to know what the program entails and how you are able to deliver, safely and effectively. Clients need to know what they are getting into before purchasing sessions from the trainer.
Session packages are important because the client pays for a group of sessions up front. In my experience, at least 95% of the people in the world are flakey, unfortunately, and selling sessions in a group protects the trainer. If the clients does not cancel their sessions in an appropriate and timely manner, the trainer is already covered.