Some people prefer a more intense approach to treatment, and certain situations require it. Most often I see clients for double sessions when they travel long distances to see me. It is easier for someone driving two hours one way to come every other week for two hours instead of making the trip weekly.
Sometimes certain people, such as trauma survivors, feel that it takes them a long time to open up and get down to the level where the hard work happens, and they prefer longer sessions. Sometimes if a client is in crisis and going through a particularly rough time, they request to come in more than once per week.
In treating a client who suffers from chronic, persistent, severe mental illness, it is often best to schedule sessions twice per week. To need treatment more frequently on a regular basis would call into question whether the outpatient setting that I provide is the appropriate level of treatment, and perhaps another, more intense treatment, at least temporarily, would be better for the client.