Somewhat a large subject, but many vehicles as well as other internal combustion engines powered items come factory equipped with magnetic devices of some sort to collect and contain metal particles. VW used magnetic plugs in transmissions back into the 60's, probably before, but I've not worked on many that date prior to circa 1962. Over the years I've seen magnet wraps for oil filters, speaker magnets on insides and outsides of oil and transmission pans, magnetic drain plugs, and even cow magnets on the farm. I feel they have two benefits, one by trapping finer metal contamination, and secondly, by allowing inspection of the debris to determine wear. I purchased a tool years ago that allows an oil filter to be easily opened, looks like an industrial can opener somewhat. This allows for inspection for debris in the oil filter, been very handy when obtaining or selling used cars. My CVZ has no drain plug, but rather a tube for oil removal, so I use a filter magnet. At oil changes, I inspect the filter and what the magnet trapped before transferring it to the new filter. Money well spent IMHO.