File / filegroup restore vs database / log restore in SQL Server
Well, here is a simple example.  You have a large database, but most of the data is history.  You go ahead and partition out the historical data onto read only filegroups.  This information is used on a regular basis for reporting but not for the actual application.  It is, lets say, 90% of your database.

If there is a problem you can restore the read/write portion much more quickly to get the application up and running.  Then once your application is functioning you can restore the read/only portion (which of course will take much longer).