As teacher who uses that phrase from time to time, it doesn't mean I've literally seen every scenario that will show up, but it does mean that I've seen and experienced enough variety that I don't get particularly surprised by much anymore. What that translates to in my classroom is that I can think on my feet. I can use my past experiences to help me make better decisions and judgments about how I handle whatever new scenarios come my way. I don't get thrown easily by the constant surprises that are apt to come up on a daily basis in my classroom. I suspect a therapist uses the phrase similarly.