Generally, it's a good idea to do that while upgrading the forum software and/or setting/resetting permissions. You can actually do those things without tuning off the board but it's an extra safety measure to ensure that people aren't posting while you're still working on something.
This is also true for installing or upgrading any addons that make changes in the user interface for posting messages. Again, not required but probably safer to do this without anyone submitting new posts.