OFF – to make it more clear to users that the view is unaltered. That then (I think) makes the process of changing video settings more clear. Otherwise you tend to get posts saying “Why does the video look all blurry”. That said, having smooth as on can often be enough to give that CRT look for many people.
ON – I like this setting in combination with scanline overlays, as I feel it gives the best representation of how I remember CRT gaming.
Horses for courses I guess.
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