Many/most economists and historians find it beneficial to understand things not by rigid or otherwise static categories and concepts, but through relations. Feudalism, insofar as it can be defined (and this is hard[1]), is probably best captured as a certain kind of relation between serfs and landlords (or whatever). Focusing on the relations between things is fruitful for analysis and research as well as being resistant to changes or transpositions in the terms in question.
It would be much harder to say that there is this one selfsame phenomena called "work" and we have done it throughout all of history. Or that we are definitely not "working" as we generate another's capital by getting brainwashed and depressed by social media. Or that the peasants living off their lord "went to work" everyday.