Not everyone who helps mental health patients in their time of need is a professional care provider. Many people volunteer their time to help those struggling with mental health issues. Some (or, arguably, all) volunteer out of a desire to help their fellow citizens in their attempts to clear the hurdle of a mental or emotional disorder. Others volunteer as a way to find out if mental health care is a profession they would like to pursue. Some who volunteer have already decided they would like to pursue a career in psychology or social work and are volunteering to learn and gain experience in the field until they're qualified to do so professionally.
As a volunteer, you'll have the opportunity to work with a wide variety of people who are dealing with mental health issues. While some forms of mental illness statistically affect certain demographics more than others, overall no individual of any age group, gender, race or even personality type is excluded from the possibility of developing a mental health disorder.
You may work with clients who suffered severe abuse in their childhoods, or who flourished personally and professionally until their lives seemingly fell apart overnight due to a mental health condition or substance abuse problem. You may interact with clients who are homeless, or who successfully hold down jobs as doctors, police officers or teachers. Some may have gotten "into the system" of their own free will, and some may have been hospitalized against their wishes. Some welcome assistance, and others resist it.
Regardless of who you are and regardless of who you help, volunteering with mental health patients is an important way to make a very real difference in the lives of those who are going through a very difficult time.