In the past, KYC was not mandatory and withdrawals could be made up to a limit of 2BTC or around $20k for level 0 users^[1]. This allowed someone to have more than one account without detection, even though the rules prohibited it. Besides HB's opinion which I think is reasonable (streamline user statistics into personal basis), the account merging feature may have been intended for users who created more than one account at the time and had unwithdrawn balances due to verification policies. We don't know what "trap" Binance planned with this.

1. https://bitcointalk.org/index.php?topic=2776379.msg29162162#msg29162162