The CustomerID can also be a "short code" (like company initials) that links to the Company table (if CustomerCompanySupport is enabled, and since CustomerID is visible everywhere, using unique numbers isn't helpful. In fact, CustomerID doesn't have to be unique as it's sometimes helpful to have members of the same company have the same CustomerID.
By default, the CustomerID is the email address of a customer for inbound emails, though they don't automatically populate the Customer database.
Like I said, this value is visible everywhere, so using an auto-increment number is going to just get in the way. If you can't figure out what to use in its place, and don't want to use email address, consider using the phone number or other customer-identifying attribute, such as CRM customer number by which you might want to later report, as this will be directly added to the ticket table upon ticket creation for this user.