It's also common in other tourist destinations. For example, many hotels close to Disney World and/or Universal Orlando require a deposit for cancelable, changeable reservations.

It's not the only reason hotel require a deposit, but one of the main ones is to discourage speculative bookings. Your example of Las Vegas is the perfect one. Las Vegas is within driving distance of over 20 million people. Imagine someone living there decides that a weekend trip in February or March sounds like fun. But, she or he needs to check with work and friends before booking. Without advanced deposit requirements, nothing stops a person from booking a room every available weekend in February and March.

(You probably already know that information. I'm just quoting you as a means to continue the conversation.)