Why is a condenser necessary in a thermal power plant instead of returning steam to boiler?

Why don't we give the steem output coming from the turbine to boiler as a feed STEEM concept insted of feed water from condenser and cooling rower. If we do so will it reduce our cost of cumbustion of fuel in thermal power plant??
Why do we need condenser in power plant??

I dont know a STEEM concept... but anyway this is done. The exit steam from the HP Turbine is used for the reheater to prepare the steam for the IP turbine. After the lp turbine the steam cant be used anymore even if there is still some energy in it., but the temperature is to low. An other concept to improve overall efficiency is to use a combined cycle which means you have a gasturbine and a steamturbine (either both sitting on a single shaft or having multiple shafts). The exhaust stream from the GT is used to heat up a boiler and the steam from the boiler is used to operate the Steamturbine.

The output is high pressure but low volume of steam as a result if you feed this to a turbine the cavitation will (eat) the turbine by high speed erosion.

By lowering the pressure and building up the volume of the flow you can run a turbine without wearing out the turbine and or the controlling valves.
Yes you do not have the same 100% of your energy but you avoid to replace a expensive turbine.

In most modern power units on this way you can connect several turbines in series every time for a lower pressure and higher flow.