If you use one email for registration on many casinos, it is good if you can label emails from different casinos. You can know what emails belong to what casino and can quickly check them out or skip them if you don't like that casino promotion.
If you are using same email then you why you need to label them as if you have any mail from the casino it will be from the casino official account and you can sort them on this basis only but if you use different mails then you neee to check which mail is linked to which casino account for checking the promotions.