Well, we don’t know what “it” is, so how would we know whether or not regular sugar would wreck it? Recipe writers generally specify certain ingredients for a reason – as we are all aware, sometimes it’s just general fussiness, but sometimes there’s a real reason!
Superfine, ultrafine, instant dissolving, bar sugar, or caster/castor sugar is generally called for if the sugar is to be dissolved, especially if it is to be dissolved in something cold. It is especially useful in meringues and cold beverages, where granulated sugar might seem a little gritty and would not dissolve as well. If the dish you are preparing is to be cooked, either sugar should dissolve completely. If appropriate to your recipe, you can also prepare a sugar syrup by dissolving the granulated sugar in some of the liquid called for in the recipe.
Off the top of our heads, we can’t think of anything that would be completely ruined with the replacement of regular granulated sugar for caster sugar. But if you have a food processor, you can grind the granulated sugar into superfine sugar yourself in just a couple of minutes, and save the agony of wondering whether you might be wrecking your dish.