dhenry:
One to buffer the output and another to limit its current.That circuit requires a 0-10v input.
So the transistor with the collector connected to the 24v is to isolate the input signal from the output and the other one is to limit the current.
Maybe I was wrong about the triangle being the op-amp.... Might be more to it than that.
I'm sure you are right about it needing a 0-10v input but can you explain.