Why does everyone like DOF preview buttons?

FilmOnly said:

I find it interesting that one can discern anything when stopped down. I have used Spotmatic equipment for years, and when stopped down, I have not ben able to notice "shadow/highlight" areas. I see only a black malaise. For outdoor shots, I use the DOF scale of the lens and hyperfocal distance almost exclusively. I find that it works with near-scientific accuracy (i.e. I almost always get what I expect). Perhaps I know my apertures very well?...

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