For my situation, it's not even so much about having a door. But having a separated work space/bookshelves (though there are plenty of others throughout the house), lights/audio gear, and generally a separated space that can be pretty messy. In times past, that would be about having lots of paper. Now it's more about having a bunch of computers/cables/etc. isolated from the rest of the house--even if, in practice, I often work elsewhere with a laptop.