Because of reuse the chemically treated water in boiler for it must be changed in liquid state so in condenser vacuum is created to condense high temperature steam by giving its latent heat to circulating water
this is a simplest definition

Dear To maximise the turbine power out put vacuum is provided in condenser. Actully first vacuum is produced by steam ejector but During running ejector is only to maintain that vacuum. basically vacuum is created due to condensation of steam .As the cooling water temprature is low enough hence vacuum is produced accordingly. Whenever steam will condense it will produce vacuum,