The first thing to note is, that even though they cannot directly claim costs, it does not mean that they cannot indirectly receive some funding for the role they have in the DN. Typically for each unit cost, there is one part that goes to the researcher and then there is the institutional part, and this part should not be seen as funding for just this particular fellow, and this beneficiary. It is rather a common pot for the whole consortium to run the project. In the consortium agreement the consortium defines how this is split. This funding can be distributed to the different partners according to their needs in the project: some partners provide more trainings, for instance, the coordinator typically has more management costs, so this funding can be redistributed, and some of this money can go to associated partners to cover the costs of them hosting researchers for secondments, or for them to provide trainings. So these are internal arrangements within the consortium (in the broader sense with the associated partners) so they can get indirectly money for their action. Of course, there are also non-financial incentives; the interest for them to participate could be transfer of knowledge or being part of a dynamic network and being associated to the research project.
For PF, direct financial benefits may not be there but there are plenty indirect benefits – scientific contributions, networking, getting experience in this type of projects, hosting events.