What is the purpose of rifampicin in the experiment?

The rifampicin is used in the experiment because its presence in the minimal media selects against the donor strain (High frequency recombination cell / a+b+c + rifampicin susceptible). If the donor strain was not eliminated it would grow on the minimal media making it impossible to decipher between the recombinant cells and the donor cells.

The recombinant cells are able to grow on rifampicin minimal media plates because they contain a rif gene which is resistant to the antimicrobial effects of rifampicin.