It is a subside to renewables, where they can make money on energy that would have been thrown away. It allows better managements of the peaks and troughs that commonly plague reneweable energy sources.
You want to help level out peaks and troughs of electricty production: Improve battery technology, don't just waste the energy generating sha256 over and over.
Before: The energy from a hydro plants gets wasted when it is night time.
After: Now they get money.
The hydro plant is now making money on something that they were just throwing away, previously, and thus it is a subsidy.
Hydro plants can usually turn off relatively quickly. They can be used as pump storage, which is an incredibly efficient way of storing energy - >70% efficiency [1].
[1] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...