Much of "Why do casino check ID?" is settled on the sign up form. Registering at an online casino takes a few minutes, and the form asks for more than a username. Name, date of birth, address, phone number and email are all used later, to check age, to match payments and to recover the account. Details typed carelessly at sign up are the most common reason for a verification problem weeks afterwards, so the form deserves a careful first pass.
Some choices made at sign up are hard to change later. The account currency is usually fixed once the first deposit is made, and a currency that differs from the bank account adds a conversion on every payment. Usernames are often permanent. Several casinos also offer deposit limits on the registration form itself, which is the easiest moment to set one. Reading these fields slowly takes a minute and saves a support request later.
Registration usually asks the player to confirm legal gambling age and to accept the terms, the privacy policy and sometimes marketing preferences. The age confirmation is checked later against a document, and an account opened by someone under age is closed with deposits returned and winnings void. The casino also checks where the player is connecting from, since a licence covers some places and excludes others, so playing while travelling can be blocked for a time.
Casinos allow one account per person, often per household, device or payment method, and a second account can get both closed and winnings withheld.
The ticked age box is checked later against a document, and an underage account is closed with deposits returned and winnings void.
Usernames are often permanent, so choosing one carefully at registration saves a support request later.
No; marketing consent is a separate tick box, and leaving it clear does not change anything about the account.