The main purpose of this line is to improve overall thermal efficiency of the plant.
ACC will have inevitably have some degree of subcooling. this means that the unit cools down the condensate more than theoretical requirement. In a typical power plant, captured condensate is pumped from condensate tank back to boiler house (de-aerator / HRSG / boiler).
This line brings some fresh steam to condensate tank and that limited quantity of steam condenses in the tank and increases liquid temperature a bit.
Typically, without a bypass line, subcooling might be in the range of 3 deg. C
but if you have a bypass line subcooling may be around 1 deg. C
This helps in saving some energy in the boiler.
If the condensate is not used for re-boiling (e.g. in sugar industry), this bypass line does not exist.