Why would I take a database offline?

Solution 1:

You can move the MDF file to another server to put it online.

Solution 2:

In development / staging environments it's sometimes useful to take a database offline to make sure that you're connecting to the right instance of the database in the application and that you don't have a jacked up connection string somewhere.

That said, in this situation it's a much better idea to have different database names for the different environments and a build process that will automatically configure your connection strings...

Similarly, I like to take databases offline for a period of time prior to decommissioning them in the (not-so)off-chance that for some reason they need to come back online. I've been bitten quite a few times by devs who have hooks that I don't know of into one of my databases when I want to get rid of it. Taking it offline much less drastic and less time consuming than deleting it and restoring it if needed.

Another thing would be as an emergency safeguard. I've had to do this before. Sometimes a nasty bug is found in your app that, even though not malicious, will still corrupt data in your database. Taking the database offline is a quick way to stop the bleeding until you get the bug identified. You can then bring it back online to assess the damage within the database.

Solution 3:

While some of the answers here may give you some ideas regarding what you can do with a database that has restricted access, not much can actually be done with a database that is offline. You can't update, upgrade, add or delete data, etc.