The phrase "principal beneficiaries" refers to the main people who gain the most from a particular situation or action. They are usually the ones who receive the greatest benefits or advantages from something. Full definition
Employees who refuse to work on their sabbath have been the principal beneficiaries of this rule (in another case a worker lost his job in a brass mill because he refused to help manufacture tank turrets) The Oregon courts followed these cases and awarded unemployment compensation to Smith and Black. (religion-online.org)
Among the principal beneficiaries of this golden age was social democracy, which had now found its raison d'être: (blog.politics.ox.ac.uk)