Let’s say you’re writing a cookbook and describing ingredients. For sure you’re going to want to be consistent from one recipe to the next. If you don’t want to confuse your readers, it’s good not to refer to one amount as “a pinch” in one recipe and “a dollop” or “a smidge” in another.
Then you look around and realize that other people are writing cookbooks and they have some standards. That’s not a pinch, to them; it’s a teaspoon to some or 5 milliliters to others. What you call a “chunk” everybody else calls “a quarter cup” or “32 grams.” So, you need to be consistent not just within your own cookbook, but with others’ cookbooks, regardless of the dish being prepared—roasts, stir fries, desserts, soups, etc.
Librarians and archivists in data repositories are learning to think like this as well. Because the data being deposited for reuse has much greater value to their institutions when the metadata attached to it are consistent at the study level, the data level, and the file level.