Why do people buy useless fish?

Why do people buy legendary (aka big) fish? Especially ones you can't put in an aquarium? I just randomly came across the Hippo Frog, and someone spent 100k on one recently. And the Hippo Frog looks like any other frog when you posterize it. I just recently caught the Warden of the Seven Hues, and decided to check it's price, and people are paying 2M apiece for it. Some guy spent 11M to buy three in February. If you didn't personally catch it, what purpose does it serve in showing it off, even if you can put it in an aquarium?

Or for that matter, why does anyone buy any fish that don't have a use? I've seen someone buy like hundreds of some random fish that you can only use for grand company provisioning missions (which you typically can only do once before you out level it), and which has no special desynth rewards. Spending like 500 gil apiece seems like a colossal waste of gil if all you are going to do is desynth them for sand and crystals.

Am I missing something here?

Regarding the warden, it's a very pretty fish and putting in a tank would be akin to any other furnishing item people pay fine gil for.
And the frog print wouldn't be like other frog prints. Similar, sure, but there's a difference.