Anything the magnet catches, saves room in the filter.
Filter can't catch anything smaller than its rating. Why do some think that filters are perfection?
Filter also has a gram loading crud rating which on some engines is pegged too early. So, the magnet frees up filter media space with whatever magnetic particles it catches, which are usually more damaging than softer non-magnetic particles.
For a drainplug, or cut open filter, one can monitor the magnet's ability freely. With visual comparison, over time, some might see that certain brands/grades/intervals can drastically increase/decrease the magnet contents over others. You don't need a UOA to adapt or change your brands/grades/intervals when you can see the difference. Also, magnet contents might contradict the UOA's false sense of security. This is not much different than varnish/sludge buildup that contradicts good UOA data too.
Magnets come standard in many differentials, transmissions(both manual and automatic), and transfer cases. I find it funny that engines and hydraulic power steering systems are often 'forgotten'.
Is the magnet collection that the threadstarter pictured excessive?..sure... but magnets are cheap or free if recycled. So, there is no reason not to if you have 1 magnet or 20. Put them to use.
My recommendation is to upgrade drain/fill plugs to magnetic ones whenever possible. Those plugs can be expensive so slapping a few magnets on a cannister filter is an easy option. Votex, DimplePlug, GoldPlug, PSR... are some good magnetic plugs.
Some of us expect or utilize our vehicles longer than others, or more abusively than others. You do whatever you want to make it last longer. The magnet is just another one of those things.
Besides drain plugs, stick on filter magnets, don't forget about the Magnefine too for your ATF/PSF.