I really dont understand what you said. I can understand how, but I need to know why this is used.
The "why" can be due to many factors. The person deciding how to build the circuit would evaluate all those and build the circuit a particular way. One of those factors might be, "that's the parts I have on hand". Or maybe it's a circuit to demonstrate the use of a particular component. We could speculate endlessly on this. Perhaps you could supply some context?