I remember using tree diagrams all the time in high school. Probably the first time we learned them was in mathematics, when they were teaching us about probability.
It's funny how you need that one single lesson, that doesn't seem that important, but it allows you to learn about so many other things later on.
Anyway, I also remember that the teacher taught us about probability by basically making us gamble on what numbers were going to come up when a die was thrown several times.
And then we noted the probabilities into a tree diagram. It took me a long time to really understand this lesson, but I got it in the end.