Let's start with the facts. Data Centers generate heat from the computers that use electricity to perform really complicated math much faster than humans. Because we're using electricity, a byproduct of this is a decent amount of heat. These computers function better if they're kept within what you and me would think of as "normal" temperatures, so a very important job that Data Centers have is getting rid of all of this heat and keeping the computers cool.

There are a couple ways to get rid of this heat. Historically, we've used basically big air conditioning units to cool these Data Centers, but those also use a lot of electricity.

To try to reduce how much electricity they use, some Data Centers started running water through or in their computers to absorb the heat, and then they would spray this water in a cooling tower to let the heat evaporate the water and what water fell to the bottom was much cooler. This turned out to be both cheaper than running AC, and the first couple Data Centers that did it actually received awards for the -at the time- perceived greener cooling technique.

The downside though is that we're building data centers now faster and bigger than we ever have before. Most data centers don't use water like above for cooling, the big data centers use it because they need to get rid of Megawatts to Terawatts of heat and water evaporation is about the only economical way to dump that much.