Why are the police involved in mental health incidents?

Often times suicidal persons are violent and require restraint. Normally when we get these calls the caller doesn't specify that the patient is petite and is willing to go with the ambulance. Since all the dispatcher knows is that someone wants to kill them self then police are sent as a safeguard for the paramedics and sometimes to affect the apprehension. Most officers these days are trained to deal with mental health patients. The officers usually arrive in pairs for their own protection and can't help it if they are burly.