It costs 80 diamonds each, and it often can only be bought in packs.
It is used to buy hearts (relationship) with the heroes who are still pending unlock at the tavern. It is used from the Hall of Heroes. This is a discount over the usual price of 100 Diamonds per heart.
Compare with buying the very same hearts for 5000 Honor Stars each, that can be gained in-game for free.
After unlocking all heroes, Beer is only worth selling.