Despite often being illegal, Soviet Union had a history of people holding a portion of their savings in foreign currency(just look up Bulgakov's Master and Margarita or Ilf and Petrov's Golden Calf for early references).
So if your local currency is in a free fall you would be well advised to convert your local roubles into hard currency.
I rememember very well the day that my father exchanged 10,000 roubles for $1,000 in the late 1980s from an American tourist. The official rate was ~1.3 dollars for 1 rouble (rouble was officially more valuable than dollar!) at the time, but the catch was you could only exchange a few hundred at a time in very special circumstances. (I believe this is how it is in Venezuela right now too, other places with currency controls).
So your hosts were making a nice gesture but also being smart in times of uncertainty because historical precedent was that their roubles would become worthless not come up.