Read Only Filegroup Perf
When a Filegroup is marked as Read-only, SQL Server will not bother with Page or Row locks on the tables or indexes contained in them. This reduces SQL Server overhead and improves performance. Since the data is not changing, index fragmentation does not occur so maintenance, such as rebuilding or reorganizing, is unnecessary. As of SQL Server 2008, you can mark a Filegroup as Read-only without having exclusive access to the entire database.
Partial Backups
Say you have a large database where a large percentage of the data is historical and therefore not changing. It would be wasteful to have to backup the entire database every night. Wasteful in time, wasteful in storage. In SQL Server you can store read-only tables in a particular Filegroup and mark the Filegroup as Read-only.
When you use the BACKUP DATABASE statement in SQL Server 2005 (and beyond) you can use the READ_WRITE_FILEGROUPS option and the Read-only Filegroups will
not be backed up.