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People born after 2000 have never seen the cosmic microwave background on their TV set.
(startrek.website)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
This is exactly why. Preventing screen burn-in may be a tiny peripheral reason also, but providing a familiar experience to chronically myopic and cranky users (i.e. boomers) is probably the bigger one.