Grist For The Mill Meaning

(idiomatic, chiefly US) Something that is useful or that creates a favorable opportunity.

Example: 2003, Peter Lefcourt, The deal : a novel of Hollywood:
  They were sitting across from each other at the bar as Lionel described his odyssey across the country in a Trailways bus, gathering material, as he put it. It was all grist for the mill, life real and raw out there west of New Jersey.