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

The idiom "grist for the mill" originates from the practice of grinding grain into flour. Grist refers to the grain that is brought to the mill to be ground, and the mill is the machine used to grind the grain. The usage of the idiom has since expanded beyond its original context and can now refer to any situation where seemingly insignificant information or material can be used to accomplish something significant.