Because often I need to highlight text in the terminal, copy it, then highlight text somewhere else (e.g. in a text input field in a browser, the address bar in the browser, etc.) and paste over it. Selecting the specific destination text I want to overwrite blows away my previous selection.

In many cases I do just highlight and right-click where I want the text pasted. But that workflow doesn't work a large portion of the time and I need a clipboard that isn't being wiped out almost any time I click on something.