Same as races/classes, monk, Fvs, Drow, WF all require some sort of unlocking through favor or buying from the DDO store. The same can be applied to enhancements. Given the PrEs for each class are hardly complete, Turbine can take the chance to give them more incentive to release them by allowing users to pay for them, or, unlock them through other means.
For example, Turbine can post "fire servant" sorcerer I, II, III, PrE for sale on DDO store. The player must purchase it in order to take it. It will behave just like other enhancement in terms of usability.
Some other example, Turbine can post "warchanter III" PrE for bards. Given Warchanter II is already released as free enhancement, this might give the player more incentive to buy the 3rd PrE.
I hope the money raised from this can speed up the development time at Turbine, and possible introduce more contents (as in raids, classes, races).