Such substances can be used to better manage abreaction and catharsis and improve the quality of understanding between the healthcare professional and patient.
The quality of the relationship with the therapist and the "self" of the therapist seems to be a significant component during the abreaction/catharsis.
Even the medical world at first thought it was a mental disease and methods of treatment, called abreaction were painful and nasty attempts to "cure" the affected people.