It seems that Vostok Games tries to attract more and more people to the game and get a lot of new players. And the best way how to do it is to give Premium keys for free. However there is a hitch in it. You will be able to play with Premium account only for one week and with 5.000 Silver in-game currency. Anyway if you like the game and try to get better weapons and experience, don't hesitate and visit VG site which is giving the keys.
The game is on the other hand for free and uses the system like World of Tanks. Therefore the Premium account is not mandatory but helps a lot.