Oh, it is really a good question. But i think you shall ask this question to joe jonas himself. I also want to know the answer. Maybe he is just nearsighted. Or he just wear the black nerd glasses for looks. Anyway, i think joe jonas knows better. If you like his way of wearing black nerd glasses, you can also try to make up yourself like him.
Well, as you know, movie stars or celebrities are always in the spotlight, and they have to attract attention from everywhere, so they gotta be different and special from time to time, creating a personal tag. You should know that nerd glasses are quite popular these days, the fashion trend made this happen, so Jonas was attracted since he finds it amazing.
The nerd glasses are so popular in 2012 now. Their appearance becomes the fashion now. Making wearers look calm, cool and wise, the nerd glasses receive a lot of popularity among the customers. That is why it attracts the favor of joe jonas. And nerd glasses are suitable for those guys who know the fashion.