Boleslavsky (White) has just played 19. f5 in response to Gligoric’s 18…Bb4.
David Bronstein notes:
“Without fearing Bxc3 which would give White a passed pawn on the c-file. This observation might come as a surprise to the reader, since White already has a passed pawn on c2. This pawn, however, cannot be considered passed with so many pieces still on the board, since it may not stir a single step without upsetting the King’s disposition. If, however, Black were to take the Knight here, then the c3-pawn, supported by the Bishop pair, could run for daylight, while the c2-pawn remained behind to shelter the King.” (excerpt from “Zurich International Chess Tournament 1953”, D. Bronstein)
To rephrase: the exchange would reduce the force left on the board, give White the Bishop pair (only considered to be the case when Black doesn’t have both Bishops himself) and still allow White the shelter of a pawn in front of his King. He can now advance the c3 pawn.