To cool the servers. Servers heat up when they compute, and many sites remove that heat through towers that evaporate water. This water is lost to the local basin: Google states that it consumes 80% of the water it withdraws in this way. In addition, water is used upstream to generate the servers' electricity, which "per query" figures often ignore.
Read the source article: How much water does ChatGPT use? A few drops per query