The same purpose as other fields. It's the method by which phenomena are investigated, knowledge is acquired, and previous knowledge is integrated.
Psychology isn't just some person asking you “how does that make you feel? “, there's a whole field investigating anything you can think of that's connected to behavior. Add neuroscience to that and it might make more sense. The two are intertwined, but don't overlap 100% (not yet anyway).
Even in the clinic, the therapies, questionnaires, tasks, interviews, all need to be tested first, validated, and confirmed. Science is used to do that. Experiments, data, publications, all contribute to what is known about us as humans.
Psychology is science and thus relies on the scientific method to advance and expand.
Thank you,
Eddie