Why condenser is used in boiler or steam power plant?

A power plant of the conventional steam turbine/generator type, whether using fossil fuel or a nuclear reactor, operates by pumping feedwater at high pressure into the steam raising units, the steam is admitted to the turbine which has various stages. At the high pressure end the rotor blades are short but they get progressively longer as you go down the machine, because the steam is expanding to a lower pressure so has a greater volume. At the back end where the steam comes out of the turbine, you will get maximum power and hence maximum efficiency of the plant by having a vacuum, so that steam below 100 degC is still doing useful work. Hence the condenser cooled by ambient temperature water from an outside source, where the below atmospheric pressure steam is condensed to water and returned to the feed pump inlets. So you may see the condenser as wasting heat, but in fact it considerably increases the overall efficiency. This steam/water cycle is called the Rankine Cycle and you will find an article on this in Wikipedia.