For this case what you must understand is that it represents each of the axes in a graph.
The horizontal axis represents the independent variable.
The vertical axis represents the dependent variable.
The use of graphics is useful because it represents the behavior of one variable with respect to the other.
Through the graphs we can observe what happens with the dependent variable when we add or remove values of the independent variable.
Answer:
They are useful because they represent the behavior of one variable with respect to the other.
