You can trade them for DEC in the Voucher Hub: https://next.splinterlands.com/voucher-hub or use them to purchase Voucher only items in the Voucher shop. So far it only has the first Voucher only promo card, which you should own at least 1 BCX of: https://next.splinterlands.com/shop/voucher
It has a great Ability called Shield Ward, check it out!
RE: Splinterlands - Surpassing 140k SPS staked on the road to 150k SPS