What are the advantages of vanilla sugar?
is there some other reason it is popular?


Yes: Vanilla beans are expensive, and once you've extracted the seeds, the husk, which still contains good vanilla flavor, is often discarded. Putting the empty husk in sugar allows you to extract some vanilla flavor that might otherwise be wasted. I usually save my empty vanilla bean pods and use them for making vanilla ice cream or some other dessert that involves hot liquids, steeping the pods in warm milk extracts the remaining flavor better than putting it in a container of sugar.