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+20 SpD. That goes from 70 to 90 SpD.
While this may not seem like much, it would have 95/105/90 bulk, paired with 100 Speed, which is pretty good. Miltank also has reliable recovery in Milk Drink, Shadow Shale, Heal Bell, Twave, etc.
Normal typing has 1 weakness in Fighting, and with Thick Fat, gains very valuable Fire and Ice resistances. This lets it pair pretty well with the plethora of 4x Ice weak and 4x Fire weak mons as well as the fuckton of Psychic mons who hate Shadow Ball.
Oh yeah, it also could be considered a hard counter to Alolan Marowak, but that's really not a big enough niche to matter currently.