I believe (i haven't done it though) that when you archive them, it puts them into an archive.pst file, and it is listed along with your current pst file. So to access the email there, you would only need to move into the other file, which is as easy as changing from inbox to sent items for example.
I would guess that it may make it a little faster to load, maybe less memory, as it doesn;t have to load in all the details all the time.
I'm purely speculating here
Confused