Why does Premiere spend so much time "relinking media" when opening a project?

When I open a large-ish project, the Progress Dashboard on Premiere spends minutes doing what it say is "relinking media".   What is it doing?  Why is this necessary?  It is technically a "background" process, but nothing really works until it is finished.

After a few mins, or 5-6 mins on really big project, it finishes "relinking" and then I can edit normally.  But my question is why can't Premiere just save the state it is when it's done and then just open quickly the next time?  Nothing has changed in my media files, why "relink"? 

NOTE: I am not talking about when Premiere cannot find the correct location and you have to point it to the right place. I'm talking about normal everyday automated stuff that happens when I open a project.