Why they would donate 10% of their power

Poorly written my bad.
Power is not to be donated it is to be sold at break even.
Concept is you created say 1 megawatt of clean power burn 90% of it to mine and run your business.
And sell 10% excess at cost to local customers.
New Jersey has this. So you build a big field and have excess power you "donate" the excess to the grid at cost "4 cents"
that is clean power for someone local to you.
The term used is donate in real world you are selling at cost.
Sorry about that.