Replaces
If existing boot is not damaged in any way then It can be re-installed. However, if it is damaged or old, I would replace It while shock is dis-assembled.
The strut boot keeps your struts protected from dirt, dust, sand, salt and other road debris. It's designed to protect the strut from bottoming out and protects other components from extreme movement damage. The purpose of the bumper is to limit suspension travel by not allowing suspension components to hit together.