It should be possible for steem bloggers to specify a section of any blog post that requires a payment to unlock. This require that the section is encrypted and the unlock key is sold on chain.
Onboards new users
When guest read a free steemit posts they don't have a huge incentive to sign up as a new steemit user. If there is a payment wall on a posts they find interesting they now have a reason to sign up.
Generates demand for STEEM
New users have a reason to get some STEEM or SBD to pay for content.
Increase blogger and investor earnings
Earnings generated using payment walls can be split between the blogger and SP holders. This creates an incentive to invest in the platform in the form of SP.