It is an anti-money laundering thing. It usually kicks in at around $3000. If you don’t use your card, they will write down your description. Some casinos are more upfront about it and ask. Some just capture your description without telling you.
If the amount you cash gets high enough, they will also fill out a currency transaction report.
A currency transaction report or CTR is required for cash transactions in the amount of $10K and above. So at first glance it might seem unreasonable for them to ask for ID unless you present tickets/chips totalling $10K or more.
However, it is a crime ("structuring") to split up what would otherwise be a $10k+ plus transaction into multiple smaller transactions for the purpose of avoiding a CTR. So I am not sure how much of this is an actual legal requirement, but I have the impression that the casinos are at least strongly pressured to start keeping track of transactions at much lower levels like the ~$3k range, so that if you later make a couple more transactions (even innocently without intent to evade the requirement) they can still know to fill out a CTR at the appropriate time.
If you refuse to show ID or a players card they'll probably just write down an approximate description, guess your height/weight etc. (one time a cashier didn't even ask me for ID or a card, which I would have willingly provided, but when she wrote down an estimated height/weight I objected because she was off by 10 pounds 
)