Why does it work to select text in visual mode and then type "d", but not when I do the same and type ":normal d" ?

I'm writing a script and it isn't working. I basically replaced the guts of my script with exec "normal d" to see if it did anything. It did not. So then I tried just typing it at the : prompt, and that also didn't work. Shouldn't that work? Or do I fundamentally misunderstand the normal command?

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