What is the purpose of vacuum in a steam tubine engine and how it works?

The purpose of the vacuum in the steam turbine is to collect vapor content in the turbine outlet thus making it water again and feed again to the boiler in which it will be pure steam then to the turbine then back again (try to learn rankine cycle). It works inside the condenser by means of water jet ejector.

to force the exhaust steam to come out of the turbine and go through the condensate pipes and get condensated back to water to continue the water loop back to the boiler

vacuum Ejector  used in condensing type  steam turbine for steam condensing at steam turbine Exhaust end and increase the turbine efficiency