WHY PUT 'KEEPERS' ON MAGNETS?
When I was starting out in Physics teaching, I was told that magnets should always be stored in pairs with two small pieces of steel on each end which were called keepers, like this:
Now, this is a good idea to help permanent magnets retain their magnetism, but I never really got a satisfactory explanation of why. In fact, more than one person I asked more or less said 'dunno, it just does' or 'that's what the manufacturer recommends'. Wikipedia explains it in terms of external fields having a deleterious effect over time, which is true - as they sit around in a lab, sometimes for months on end, waiting to be used, people will be removing and replacing other magnets which are stored nearby, and this will distort the field and de-align the domains.
However, an additional reason that didn't occur to me until much later is that each magnet's field lines diverge as they pass through the faces of the magnet. This has the effect of slightly encouraging the magnet's domains to diverge which reduces their degree of alignment and, hence, the overall strength of the magnet. If you place two magnets nose-to-tail with keepers, as above, the magnetic field lines are more constrained to be parallel and so the effect is reduced.