That option is primarily for those who have multiple accounts with the exchange and might want to get one deleted. Instead of deleting it, the exchange offers you an option to merge both together, which they will bring both your trading and deposit history from both and merge it together, completely wiping one of your accounts off and leaving everything to one.
But if you only have one account and want to delete it, then the best thing for you to do is to use the first option; you have no business with the second option..
AFAIK Binance doesn't allow multiple accounts since KYC is tied to just one person. In that sense, it's generous of them to offer an account-merging feature rather than simply banning or suspending the extra accounts outright
...But the second one, I think Binance just want to know if the person is still having another account with them and not because of merging the accounts together.
It's basically in the check box, to merge multiple accounts, since it's not allowed just like KYC is tied for one person only.