Wifi provides a connection to internet devices through fixed Wifi hotspots, perhaps your favorite restaurant down the street or the bookstore in a local shopping center.
However, a Mifi is a hand held router which is its own mobile hotspot. Mobile Mifi is your very own solution to internet connection when you need it. When you are on the move, this is mobile internet connectivity.
In a 2 words - Cost and Felxibility. When you sign into a long term contract with the ATT, Verizon,T-Mobile, Sprint - you are locked into that contract - and in most cases you are paying a LOT MORE then the same coverage on the same networks. When you go the no-contract route, you have easy flexibility to get more or less bandwidth and you only need to commit to 1 month at a time.
AT&T and T-Mobile network is based on GSM systems which do use SIM cards. Verizon and Sprint don't use SIM cards when using their 3g networks - BUT with the new faster 4g LTE systems - they also will require a SIM card which is installed in your MIFI device.