These are easier to remove than a frame to make space for removing the
other combs.  Use with 9 frames, they keep everything tight and neat, and
probably reduce burr comb construction. They temper the temperature drop at
the sides of the hive.
 
But they are another piece of equipment to build, worry about and scrape off.
IMO. Some simply-constructed ones warp badly, too.  Otherwise granted they
can be a plus. They aren't necessary, though, really.