They use the GPUs for Cryptocurrency mining, which rewards them with coins. It used to be very profitable, but now the competition is tight and margins are slim. Most bigger coins have long since been unprofitable to mine with GPUs, but some purposely chose algorithms to avoid more industrial mining, in efforts to keep mining decentralized, by being done in the hands of the average Joe.

The mining requires mostly doing lots of one cryptographic algorithm with specific inputs, such as Scrypt, SHA256, and a bunch of weirdo ones that have since sprung up.

The GPUs are uniquely well setup for such a task, by naturally doing tons of parallel processing, compared to CPUs. A GPU may have thousands of compute cores, normally used for pumping vertex and color data through transformation pipelines for 3D rendering, but they do just as well as parallel Scrypt hash processing cores.