Substituting the value of "X= 1" in the last step in the figure, we can see the reason intuitively from the following figure (Y replaces A in the figure, and the red part is the value range of X and Y respectively):

because

Therefore, in the range of X, when x= 1, the range of Y is the intersection interval corresponding to the X interval, so it is substituted into x= 1 for evaluation.