I don't know. They are convenient, they are a justified source of noise such as fans, they are stealthy, they readily available for a few pounds if not free, noone is going to walk into you room and curious decide they want to look in your computer like they might a wardrobe or closet, it produces bud. Not everyone has a license or a big spare room or even a spare cupboard, not everyone has the privacy they'd ideally like. I mean hell, look at me and my big 600w hydroponic cupboard, result of that is unless they're gonna still be drunk and unconcious at about 8am then no girls stay in my bed overnight

"why is your wardrobe humming to me??" Only with my computer on is there any stealth in my room, without it i'd be busted faster than you can break your fingers.
Bigger isn't always the best in every situation, sometimes you gotta make tradeoffs and find a balance.