As I understand it : When you open your Steemit account for the first time you are sponsored x amount of SP. If you're buying an account ( you want a second account of you lost your password and need to open a new account. Don't ever lose your password) the minimum is $6 but you start with an sp balance of at least 5. As you progress through the ranks you build up your own sp and you no longer need to be subsidized by Steemit. Those funds are then used for other new accounts. You can check it out on Steemd.com/@yourusername and you can see when the Steem bot ( Steemy I think?) removed it's delegation to you.