There's another reason for using infused sugar, and that's complexity - preparing the sugar would let one deal with a single more complex (and more predictable) ingredient, instead of more than one individual ingredient.
So, if I had, say, orange peel sugar - I don't need to have orange zest on hand (or have to dig it out of wherever it may be hiding) if I want just a bit of orange flavor. Not to mention the difference between fresh and dried zest... I'd think a sugar made with fresh peel would be closer to fresh zest, than the dried zest from the spice rack, which is even trickier to have on hand exactly when one needs it.
And I could make a larger batch of orange sugar, and use it for several things, making it even more convenient to add just a hint of orange flavor wherever I'd like. If I wanted to use just a touch of zest, I would have to measure carefully, and mix carefully, each time... and being a bit offset in ratios and amounts of intense spices like zest could produce very noticeable differences, moreso than being a bit off in a bulkier and subtler ingredient like infused sugar.
Additionally, I could well make more complex infusions - like a citrus sugar with lemon and lime and orange zest, or an orange-and-vanilla combo, or, well, most any combination. This is again, a convenient and predictable flavor profile, instead of digging out multiple separate ingredients, and measuring a precise ratio for a very small amount (to get a subtle flavor). And with more complex ratios, it's easier to get a small but noticeable change, like this recipe tastes a little more lemony than citrusy, must've dropped in a half-pinch more zest. Or oh, this one's heavy on the vanilla, can hardly taste the orange.
Then, too, especially for more complex combinations, flavors that hold together for a longer time taste different from a simple mixture, when the flavors are given a chance to meld and mellow - the same basic concept behind long-cooking foods like beans or stew getting better for sitting longer, or other foods that age well together. A sugar infused with lemon-lime-orange, will come together with a single taste that's more smooth, or maybe more overall-citrusy, than tasting the individual hints of lemon and lime and orange zests in a different recipe.