تعریف و معنای "grist to the mill"

What is the origin of the idiom "grist to the mill" and when to use it?

The idiom "grist to the mill" originates from the practice of grinding grain in a mill. "Grist" refers to grain that is brought to a mill to be ground into flour. In the past, millers would grind grain brought by local farmers and keep a portion as payment. This phrase came to mean any resource that is useful or profitable, as any grain brought to the mill added to the miller's business and profit. Over time, it evolved to refer to anything that can be turned to one's advantage or benefit.