The Life Model and the Life Planning Workbook can be used by therapists, counselors and practitioners as a process tool. At that point, you'll have specific goals and issues to discuss with the client, as well as a vividly described picture of a desired outcome.
People always know what they DON'T want in their lives, based on their own experiences.
But when they're asked what they DO want, they often don't know. It's harder to describe something they haven't already experienced. In terms of Erikson's model of development, the Workbook is most useful in the Adolescent and Adult phases of development. It isn't intended to be therapy in itself, so it isn't based on any particular school of thought. It can be used, however, as an aid during therapy.