What is the function of dearater?

to remove the air & gasses for heat transfer purpose. 

to remove the dissolved oxygen from the water before feeding it to the boilers

The function of Deaerator to remove dissolved gases like oxygen from the feed water to steam generating boilers. it is very important because the dissolve oxygen cause the corrosion and demage steam system.

To remove dissolve gases into the feed water by deaeration process.

To remove dissolved gases from feedwater- main reason of corrosion 

dearator is a hot return & feed water tank ,it is one of the most important components which used in steam boilers systems , it reduce the chemical adding and softening of the feed water to the boiler .remove all the useless gases which me harm the inside boiler metal meanly oxygen to prevent scaling  and corrosion,it helps to feed the boiler suitable temperature of feeding water to reduce the oil consumption   

Deaeration strips the dissolved gases from feed water as well it does increase the temperature of feed water when Steam is used. Solubility of gases decreases with increasing temp. This high temp also support removal of gases.

The function of a deaerator in the steam power plant is to remove the dissolved gas in the system feed by the condensate pump, soft water or a tap water supply. From there, it goes to feed water pump. Where it will distribute it to the boilers.