Perhaps this could be made a setting, so players can decide whether spectators should be able to see the revealed card or not.

Hmm, I guess it could be a setting, I wouldn't want it as default to avoid spoiling the game.

Not entirely sure how to implement it in the lobby though, as it would be a sign that only makes sense when the other setting is active. I guess it could be hidden like multi player settings when inactive?

Still a bit weird to communicate I guess.

What's the thought behind it, or what would be the advantage of seeing the items as spectator?

The sign being hidden would make sense, yes. The main thought behind this is the fact that spectators would have an overview of the game that is different to what the players can see. In many E-Sports for instance, spectators can see the whole game progress and have more insight than what each individual player/team has due to the nature of the game. In regards to Bingo, knowing the card could bring a different and perhaps more interesting viewing experience to spectators. Simply put, they can laugh about players missing easy items, or get excited when a player gets close to collecting one. They may even give hints if the players so desire.