The person-environment-occupation model was developed as a means of drawing connections between people, environment, and occupation. Specifically, the model was developed because there was a lack of literature connecting occupational theory to clinical application of the interaction between people, the environment, and occupation. Even more specifically, the model shows that there is an interdependent relationship between a person and his/her environment. The model is useful as a framework off of which an occupational therapist can base reasoning and analysis in a clinical setting (University of Cape Town, "The Person-Environment-Occupation Model").
Many different peer-reviewed articles can be found in scholarly journals discussing the importance of the person-environment-occupation model; however, online access to them is limited. Nevertheless, you should be able to access such articles through your school library's databases. Start by using as a keyword search "person-environment-occupational model" and see what you come up with. You can also try a keyword phrase such as "usefulness of person-environment-occupation model" and try substituting the word usefulness for words like effectiveness or responsiveness instead. One such article you should be able to access discusses using the model to "target interventions" and to "implement interventions at different levels"; the article was published in 1996 in the Canadian Journal of Occupational Therapy, titled "The Person-Environment-Occupation Model: A Transactive Approach to Occupational Performance," and written by Mary Law, et al. If you can better see how the model is used, then you should be able to also explain its usefulness. The article is a bit outdated, but you should be able to find more current ones in your library's databases. Ask your research librarian for help if you need it.
the Canadian Occupational Performance Measure (COPM) is a tool designed to be used by occupational therapists to help occupational therapy patients detect changes in their own occupational performance abilities over time (Mary Law et al.,"Canadian Occupational Performance Measure"). The tool is far more patient-centered than other evaluation tools and caters to the fact that not all patients will have the same occupational performance goals or respond in the same way to treatments.
You should also be able to do the same as shown in the paragraph above to search for literature discussing the usefulness of the COPM as well. In fact, as recently as 2011, a study was conducted and published in the Journal of Rehabilitation Research & Development exploring how well the COPM works to help occupational therapy patients "identify, prioritize, and evaluate important issues [the patients] encounter in occupational performance." Using a series of questionnaires and evaluation techniques, the researches concluded that the COPM is an effective means of "detect[ing] changes in perceived occupational performance issues." The study is titled "Responsiveness of the Canadian Occupational Performance Measure" and was written by Eyssen, Isaline, et al.; you should also be able to find similar studies evaluating the usefulness of COPM if needed.