All online casinos offer bonuses or other benefits for signing up as a new user. Sometimes you don’t even need to make a deposit in order to receive them, and even if you do, the match bonus might still be worth hundreds or thousands of dollars. New players are often also entered in freeroll tournaments.
The money spent by the casino on all of these benefits is worth it, but only if each player gets them only once. If a player registers over and over with different names, the casino loses money. To prevent it, online casinos are able to identify the computer used to register a new user. Even letting someone else register from your computer means fraud. It’s explained in terms and conditions, which everyone needs to read.
The casino software you download has ways to collect information that can identify a particular computer, such as IP address, network card’s MAC address, CPU serial number and version, hard drive serial, etc. Every time a new user registers, the casino checks if the information matches with anything they already have stored. If it does, the registration stops and the player gets banned for fraud. It doesn’t always work, but it’s effective enough.
If someone wants to avoid it by using another computer, they will probably still stumble upon other identity checks that include the address, the bank account and/or other information.