Display models. They're great show ponies, When set up right and with a little flair, they look very impressive to your average punter, they make it harder for sticky-fingered attendees to swipe shit, and they don't have to run hard or long-term. I can't think of any others worth mentioning.
There's also a number of other problems - For one example, a lot of people do dumb shit like assuming that the thermal mass of the oil is enough to keep their computer cool, when you really need a pump-and-radiator system(And the bigger the radiator, the better) to cool the oil. Which has also led to a bunch of destroyed furniture, carpet and the like, because fun facts, the specific heat capacity of most mineral oils is higher than the safe temperature for one very important component. No, not any of the PC components - I mean the acrylic that 99% of the tank/cases you can get are made of.
And when that hot Acrylic, which is also under pressure from the weight of the mineral oil, bends and cracks from the stress, the best case scenario is a nasty leak that requires complete disassembly and replacement. The worst case scenario? Well, let's just say that Bob Ross wasn't the only dude who could make an impressive waterfall in oils.