This site is down for maintenance

Whhy do some websites need to spend several days down for "maintenance"?  What are they doing?  Do the adminitrators need to take out the website and put it on a ramp while they inspect it for wear?  Surely all they need to do is make all the updates offline, and then spend a few minutes uploading the changes. 

Clerarly there's more to it than that, but what are the maintainers doing?