A good use case for berries would be adding a feature to trade in berries for another currency (new currency will be used to purchase in-game items; weapons, amour, keys, etc..) this way you need lots of berries to exchange (lets say 1:5 ratio for new currency as an example my point is you need lots of berries and not a little amount as that would be too easy). To make it more difficult add in kill counts; you need lots of berries, new currency and kill counts to receive the in-game store items. Players will try to use irl money to buy berries but they still need to play the game for the kill counts! Making it even more difficult to get would be making the in-game items untradeable so players HAVE to grind berries and kill counts to receive such amazing awards.
Berries earn a use case
New currency for in-game items
Cant use $USD to buy your way into in-game store items