The term useful Jew was used in various historical contexts, typically describing a Jew useful in implementing an official authority's policy, sometimes by oppressing other Jews.
In 1744, Frederick II of Prussia introduced the practice of limiting Jewish population to a small number of the most wealthy families, known as Schutzjuden ("protected Jews"). The first-born son in such families inherited this privilege; other children were considered useless by the authorities and had the choice of either abstaining from marriage or leaving.
Following the establishment of the Pale of Settlement by Imperial Russia, only "useful Jews" (Russian: полезные евреи) were allowed to live outside the Pale; these included Jews such as wealthy first-rung merchants (Russian: купцы первой гильдии), persons who had received higher education, cantonists (after serving their full term in the army), and some other categories.
In Israel and among its supporters, along with the term "useful Jews", the term "useful idiots" is often used to describe anti-Zionist/post-Zionist Jews who stand against the Jewish state.
↑(in Russian)The infamous AZCSP (in Russian) Prepared by Moscow Bureau for human rights. Contains excerpts from June 6, 1983 AZSCP press-conference from the press-center of Soviet Ministry of Foreign Affairs. (sem40.ru)