Originally Posted by mithrilG60

It's done more to improve/change the nosing experience of the whisky than it it to effect the taste. Basically adding water releases more aromatics from the whisky by slightly diluting it and that dilution also reduces the deadening effect that alcohol has on your sense of smell which allows you to discern more. The higher the alcohol content of the whisky the more pronounced this effect is. However with high ABV cask strength whiskies it can also be very helpful in reducing how much the alcohol deadens your taste buds. In that scenario it obviously impacts both taste and nose.

With many cask strength whisky you want to add a surprising amount of water, for example with Aberlour A'bunadh which is ~60% ABV you'll want to add water at 2:1 whisky to water ratio to reduce it down to 40%. The difference in that case is very dramatic. With many you won't necessarily see a huge difference. Typically I'll add water to a new whisky the first couple times I try it and then if there no real difference I won't bother after that.