Hazelnuts are naturally sweet and full of nutrients.
A Brief History Lesson
Hazelnuts come from the corylus avellana, a tree which originated in Europe and western Asia. It first became part of organized agriculture some nine millennia ago, in what is now Scotland. Those ancient people didn't have much in the way of meat up in that part of the world, so they relied on nuts to keep them full. The nut's fatty and nutritious composition would have kept them solid and healthy. Imagine that, 9,000 years pass and people are still eating the same things, albeit with a little extra sugar nowadays - though hazelnut coffee probably doesn't count as part of the paleo diet.
These days, hazelnuts are grown all across the globe, in Turkey, Poland, the U.S. and China. Hazelnut spreads and candies are the chief uses of the nut and public demand for the unique flavor just keeps growing. You can also grab a cup of hazelnut coffee in just about every restaurant and cafe across the globe, but none of those made with sugary syrup will be as good as the cup you brew at home.