I was thinking for setting up brand new tanks for newbies. And perhaps for information and education purposes of setting up second tank/filter. Newbies don't have a tank at all, never mind a second quarantine tank! I think a bigger and more common problem is people who don't know better, get frustrated waiting for cycling and add fish regardless and that's how so many fish are needlessly killed by new fish keepers. When there new 55 gallon tank is a week or 2 old they throw half a ton of new plants and 20 fish. Regardless of quarantine issues those fish will probably die of ammonia or nitrite poisoning but would stand a chance of surviving if the filter had some mature media in it. I just think GOOD fish shops could do more to help newbies seed their new tanks and cycle quicker to prevent this.
Marine aquarium fish keepers buy live rock for similar purposes. What's the difference?