I think it comes from the old steemit signup process, because the accounts I have made via steemworld only have the 4 keys I actually need. The master password seems to be one steemit issued so you can actually then reve4al the master key in the interface. I could be wrong but I also think that it can get a lot of people in trouble all because it is not made obvious. Then again my account is 2 years old they may have told me about the password back then :)