I view dirty vocals as an aesthetic choice rather than emotional, they pair better with distorted guitar tones, and with metal, the focus is more on rhythm than melody anyways, so a more percussive vocal style makes sense.

Edit: + I kind of like the fact you usually can’t understand the lyrics straight up, because it’s easier to ignore them if they’re bad and just enjoy the instrumentals, and if they’re good, you can still follow just fine once you know them