Since casinos are not giving any kind of free money and they have a house edge that guarantee the win for them in a long run no matter where do you bet or from which account, i.e. They aren’t either losers or winners. It’s usually used to prevent spamming by bots. The idea is that why would a bot use dual account while they can change seeds and do everything from one account?!
I’M afraid there is a something we don’t know about this If you ever visit scam accusations section, you should have seen atleast one problem from this kind from a very trusted known casinos which looks weird for me.
The casinos do promotions occasionally and those who have multiple accounts can abuse those promotions easily. I can't see any other reason to be honest. Maybe using multiple accounts make the tracking harder for the casino because they are logging the IP's of each player and if the guy uses multiple accounts and and different internet lines, it might create some legal problems for the casino.