To help your teammates pick the best cards, and to be eligible for current Team War. Once you vote on at least one card, you become part of the Team War for your current team. Another important aspect of Voting Days is that all 12 pairs of cards should have at least one vote for the team to proceed to Upgrade Days. If a pair of cards has no votes, the Team is disqualified from remaining days of Team War, because it won't have a valid Team Deck.