Comment by nunobrito

The water is reutilized, a big reason is the difficuty to filter new incoming water because of impurities and uncertainty about quality (e.g. winter times make the river water very muddy and difficult to filter).

Second because is because adding water is a cost, whereas reuse existing water is simpler and saves money. There are always losses of water, however these are neglectible.

Not mentioned here but for more extreme cases of devices cooling is done with distilled water (zero minerals) and the whole device works submerged under this water, the hot water isn't thrown away because it distilled water takes a lot of effort to remove the minerals and effort to keep them out, so the closed loop is very efficient.