[dba-SQLServer] Turn off an index

My DBFH large table never changes, it is read-only.  I create cover indexes for sets of related 
fields used in where clauses.  Why would I turn on / off an index?  Is there any advantage to 
leaving an index "turned off" for a read-only file or is this one of those "while you are updating" 
kind of things that I don't need to be concerned about?

-- 
John W. Colby
www.ColbyConsulting.com