The Ranger Card Doubler exists for players who want to double-up on their card sets (aspect cards, personality cards, specialties, and backgrounds). For example, if you want to play a campaign with two Rangers who have the same background, or two who have the same specialty, or even the same role, you'll need to pick up the Ranger Card Doubler.
It's also good for groups of 3 to 4 players if--as that third and fourth player--you'd like more options than whatever the first two players haven't chosen. It's by no means necessary, however. The core set fully supports up to 4 players.