Cashier Asking for ID on large cash outs

[ConsciousOne]

I was playing in the Bellagio today, and when i went to cash out, someone in front of me was cashing out about 3k in chips. She asked him for ID when he cashed out (he looked around 40). He was confused by it, and i noticed and asked if it was for tax reasons; she responded by saying "im not legally allowed to say why". The only other thing she said was that they had to keep track of it, and she coudlnt say why so people didnt try to get around it.

Does anyone know why this is? I am assuming its for the goverment to keep track of if people are paying their taxes on their winnings?