Let's see the picture below.  The field on the other side of the plate is almost nil.  This is because the plate has diverted the field, causing a lot of it to flow within the plate itself instead of in the air. 

This is sometimes used to help shield a speaker from your TV set.  Placing the speaker in a box lined with sheet steel prevents the speaker magnet from messing with the colors on your TV screen.  Or, the speaker manufacturer may place shielding around the speaker magnet itself - much smaller and more effective.