system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do nuclear power stations need cooling towers instead of using the excess heat to create more electricity?
gpt
Cooling towers are actually more common in coal fired plants, though they can be used in nuke plants also. Both types of plants use the energy to heat water and turn it into steam and the steam then powers a turbine to create electricity.

Once the steam has done its job of powering the turbine, you then need to turn the steam back into water by cooling it. That's what the cooling towers are for. Otherwise you would need a constant supply of fresh water to turn to steam rather than being able to use a closed system with a finite amount of water.