A MiFi device uses a wireless 3G or 4G signal to access the Internet and then transmits a Wi-Fi signal that can be shared with multiple devices. Nowadays we carry a range of devices that want to be connected to the web: notebooks, iPads, smartphones, even cameras and portable media players. Your own personal WiFi hotspot which you can carry around anywhere and use with any WiFi enabled devices is very useful.
However, these devices are a bit late to the market, as Wikipedia points out, various smarphones have started offering the functionality of turning them into a mobile hotspot:
Android phones running Android 2.2 or later can create a personal hotspot.
No need to carry around and charge two devices and pay for 2 data plans. These devices will be around in some pockets till awareness about mobiles and apps that provide this functionality increases, after that it’s game over.