These tool supports the whole software live cicle from modelling of the application development to the persitent storage of program data. There exists the possibility to generate objects and their access to the database you need for development. An import function reads object models (for example Rational Rose) and generate code out of it.
In an object oriented environment like Java or Smalltalk the access to a relational database is connected with a lot of work for programming. Objects has to be written in flat table struktures of the database. After then you have to access them and convert again to objects. Persistence frameworks like POF minimize the effort of design and programming.