To be clear and upfront: You Do~Not Need This Game-Item!! You can easily play the !!FREE!! Card-Game without a Game-Modem!! It just 'really' makes the Game-Play more 'comfortable' and it is therefore a Luxury-Item that is optional to players who can spare some coin.
Yes, There are also advantages to playing with a Modem, such as; When playing Base-Rules your Hero-Card is placed 'Open' _Face-Up_ in front of you and All-Other-Players can read your 'strength' from the card. When you play with a Modem: The Card is placed Vertical in the Card-Slot; Facing You, like you see in the picture above... Which means your enemies see the 'Back' of your Hero-Card and cannot read your strength, thereby losing the ability to 'pick' the weakest target on the battle-grid as easily as in the card-only Base-Game.
In short, the Definition of a Game-Modem is: A Luxury-Card-Holder with a built-in Life and Win-Counter!
—description from the publisher