You can tether your computer to your phone and use the phone's 3g. I'm not sure I understand what you're asking. The benefit is that you can use the internet on your laptop (or any wifi capable device) anytime that you have your phone handy. I prefer to surf the internet on a 17 inch screen over a 3.7 inch screen.
If you are wondering about the cap, it has been yes and no for a while. I try to be smart about it. Im probably not gonna go crazy and get rid of my cable modem and strictly tether off of my phone. I use it sparingly.