Ok, it seems that you are very interested in that musician. As far as I know, he is quite famous in the music circle and he likes to wear different types of eyewear and sunglasses during his performance. So, what he likes the best is called wayfarer sunglasses, which could make him appear more awesome and amazing, more like a singer. You could get lots of wayfarer sunglasses at Amazon or Walmart vison center, where there are more options and the prices are affordable.
Those celebrities love sunglasses deeply than other people. The main reason for this phenomenon is they need to be cool, then they need to hid their sleepy eyes caused by the endless flying.
Julian Casablancas is a talented singer and song writer, he is really busy and he always gets sleepy eyes, so sunglasses are the necessity for him.
No matter what, we love him for his awesome music. But we have to admit, sunglasses do make him special.