1
general::
Something that can be used to advantage, as in These seemingly useless data will be grist for the mill when he lodges a complaint. This expression alludes to grist, the amount of grain that can be ground at one time. [Late 1500s]
American Heritage Idioms
2
general::
Phrase(s): grist for the mill [and] grist for someone’s mill; grist to the mill
Fig. something useful or needed. • Bob bases the novels he writes on his own experience, so everything that happens to him is grist for the mill. • Ever since I started making patchwork quilts, every scrap of cloth I find is grist for the mill.
McGrawhill's American Idioms And Phrasal Verbs