Sometimes while in maintenance mode we still want our visitors/users to be able to at least browse the board. So we think that it will be a great addition to have this capability in maintenance mode to serve the read only data to the site visitors. Masking all sorts of database write requests/forms and a global announcement regarding the maintenance. Log in sessions may also be disabled for that period.
I am not very sure how archiving or thread caching is done in XF but just throwing ball in the air if it is possible to serve those static files without any database interaction at all.