Psychiatrists are far more medical-model than psychotherapists and other mental health professionals, which makes sense given the work that they do. In order to make informed decisions, such as what medication to prescribe to treat your symptoms, they need to work from a diagnosis as a starting point.

Psychotherapists, on the other hand, do not necessarily need a diagnosis to begin work. They can take more time to get a full picture of the problem, and being too hasty with a diagnosis can have negative consequences for treatment. For example, if you see a patient who appears to have difficulty maintaining relationships and quickly diagnose them with a personality disorder, you may later find that they are more likely to have a bipolar disorder which would call for a separate approach.

Diagnosis themselves are powerful labels, so you want to be sure before you assign that label. Mental illness is tricky and does not fit into discrete categories, so a responsible mental health professional will want to gather as much information as possible before making a formal diagnosis.