Not everything that goes into a toaster is the same thickness. The extra heat is to get a good toast on something sliced very thin. Or you might just want some burnt toast.
I use a super mild setting for things like bagels, which are very thick and would quickly burn with a normal setting. A normal setting for things like toast. And a higher setting for poptarts because they are thinner than toast.
Apart from just thickness, things cook at different speeds. A wheat toast is going to toast at a different temp/time combo than, say, a super dense beer bread. A high temperature could be used for something that was really moist but you wanted toasted. Like zucchini bread.