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In "The New Penguin Russian Course", M.J. Brown (1996), a tourist in Moscow asks:
Who was Dzherzhinsky?
using "кем" (with whom) as opposed to "кто" (who) -- Mr. Brown did not give an explanation - why would Russians say
with-whom-was-Dzh...?
to mean
who-was-Dzh...?
In English you could say “Dzherzhinsky was a revolutionary”, while in Russian this would be «Дзержинский был революционером» – in the Instrumental case.
Perhaps it is a little easier to see in the following sentence: “Dzherzhinsky worked as a politician” and «Дзержинский работал политиком» – in English you use a preposition, while in Russian you use a specific case for the same purpose. Going back to your example, in English with “to be” you don’t need a preposition, while in Russian you still need Instrumental with «быть» the same as with «работать».
It is hard to give an answer to “why”, that’s just how governance works in languages: unfortunately you have to simply learn which verbs require which prepositions (in English) or prepositions/cases (in Russian).
