Sure, I understand. You can also do that by reversing the order of the two operations (paste into the document, then select and delete the unnecessary text), or you can use both selections (select the text to be copied while holding down alt, then select the text to replace while not holding alt, and middle click from the secondary selection by holding alt).

But that’s not really the point; the point is that it’s nice to have both mechanisms available. Some people will find selections more convenient, others won’t.