Data centres (or centers) run 24/7, consume large amounts of electricity & generate large amounts of heat. Instead of wasting that heat, Stockholm is harnessing it through the use of heat pumps and then selling it into the open district heating system. Apart from capitalising on a traditional waste product, the re-use of the heat reduces the energy needed by the district heating system, in turn reducing energy consumption and emissions.
https://www.strategy-business.com/article/How-data-centers-can-give-back-to-society