With earlier version of the foreign function interface it was not possible to create new foreign types that encapsulated the functionality of existing types. The only way in which types could be abstracted was to create "wrapper" functions that filtered the uses of a given type. The FLI contains the ability to encapsulate foreign types, along with the ability to create parameterized types. This enables you to easily create more advanced and powerful type definitions.
LispWorks Foreign Language Interface User Guide and Reference Manual - 7 Dec 2011