Not everyone has the space for a decent speaker setup. An open back pair of headphones will work anywhere as long as the environment isn't too loud. Another reason could be if you are living in an apartment with neighbors who won't be very happy with your speaker setup blasting sound through the walls into their apartment. Open back headphones do leak sound but in a much more localized area. Also, you can get a great pair of open back headphones for $200 or less and then with a $50-$100 amp have amazing sound. I haven't looked closely into speaker setups but I feel like a good speaker setup is going to cost more than that.