This relic's ability allows the player to play more cards on turn 1 after entering a Rest Site, which means it is more likely for a combat to end much faster than it normally would. This give the player an incentive to route towards Rest Sites. Since boss combats are always preceded with a Rest Site, this relic guarantees 2 additional Energy at the start of boss combats (barring some exceptions, see the Secret Portal event and the Ascension 20 modifier).
It is important to note that entering ? rooms or Treasure Rooms does not mean that the effect of the relic is wasted, as the Energy gain only triggers when entering a combat.