I thought it was an anti-cheating measure. It makes it harder for the dealer to slipping an extra card in when they are cutting the deck, and makes dealing from the bottom difficult.
Mostly to block players from seeing the bottom card. That would really give a player an advantage. They also use them to mark where the deck needs to be reshuffled.
Oh. That makes sense. I was imagining all sorts of weird reasons why they use that card. It's really simple when you consider that it's an anti cheating measure.
It is an anti disclosure (not cheating) measure to prevent you from seeing the bottom card. The other one is used to determine how deep into the deck they will deal and is usually set by casino policy (1.5 decks from end). The more cards after this "cut" card the weaker a card counters game becomes, so it is a counter measure.