They will often have the most interaction with patients, and are key for any mental health team to have a level of trust and understanding with patients.
To become a mental health nurse, you must have already completed your nursing degree, during your degree there would have usually been modules and courses catered specifically toward mental health nursing, and of course, further training is given once leaving university.
Psychiatrists rely heavily on a mental health team, as they often have very little time with each patient individually, as they are often responsible for a large quantity of patients.
Therefore, other members of the team must know what type of treatment is needed for a patient, and suggest this to the psychiatrist, who as aforementioned, has the authority to prescribe said medication.