Gives the ability to have 2 different sim cards in the phone at the same time, hence having 2 different numbers using the same phone rather than having to carry 2 phones for those of us that have 2 numbers - I got a samsung duo a few months ago because I was buying a new business that had existing customers calling the old number, but have my own number too. So rather than carrying 2 phones, got the one that takes 2 sims, and by now hopefully all the old people are using my new number... plus it gives the ability to use 2 different networks. There are issues, such as if you are on a call on one line it will go to voicemail on the other, and with my phone at least if you want to change the primary sim you need to reboot the phone, but they are a good idea in the right situation. Of course, I could have always used call diversion instead...