Gland Steam condenser is a device in which the exhaust steam from steam turbine is condensed by means of cooling water.
The main purpose of a steam condenser in turbine is to maintain a low back pressure on the exhaust side of the steam turbine.
After releasing from nozzles, the steam has to expand to a great extent for converting available energy into it to usable mechanical work.
So, if the steam after doing it, work, does not get condensed, it will not give required space to other steam behind it, to expand to its required volume.
Condensation of steam in a closed system, creates an empty place by reduction of volume of the low-pressure steam.