In most systems, the condensate is treated with additives to reduce corrosion. If condensate is dumped down the drain and fresh makeup water is added, additional chemicals would be required to condition the water. Dumping hot condensate is thermal pollution and the temperature at which the water may be dumped into the drain is regulated. Also, fresh water is not free. Every gallon of condensate dumped into the drain must be replaced with one gallon of fresh water. For these reasons, condensate should be captured and reused.
The simple steam system did not remain simple. In time, the steam and condensate were separated into different lines. The one-pipe steam systems became two-pipe systems. Buildings and systems became bigger. Horizontal runs of return piping became longer. As a result, the installations lacked the vertical column height of water needed to push the condensate back into the boiler.