When you opened your steemian account, you were given a randomly generated sequence of letters and numbers as your password. You were the told to save it offline so that it wouldn't ever get lost as it could not be recovered. This is to optimize security and protect the steemians as best they can from people seeking to steal accounts. So when you try to transfer steem or SBD, the system requires the password so that incase anyone is trying to use your phone to steal it, they won't be able to without that password. And it's much easier to steal a randomly generated password than it is to steal one that you could have come up with.