Sometimes that's the reason. It also helps the T write down notes between appts, and go to the washroom and answer phone calls and stuff. It's also useful if the client is in crisis and needs a few more minutes to get settled before they have to leave.

I know at my school, they start classes 5 minutes after the start of the hour (or half hour) and end 5 minutes before the end of the hour (or half hour) ... to allow for students to get from one class to another.