Why is MacOS window translucency enabled by default?

IMHO translucent windows that let background windows and desktop pictures bleed through are annoying. Consider the difficulty reading sheets of "real" paper used for documents such as letters or instruction sheets if they were translucent? It also makes user guide creation difficult because every user will experience a different window appearance. Window translucency is a gimmick that demonstrates a "cute" technology but is of no essential utility and it should be disabled by default.
Why is MacOS window translucency enabled by default?
 
 
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