Usefulness is the noun from useful: the quality of being useful, or how much use something has. You talk about the usefulness of a tool, a rule, a website, an idea. It often shows up when someone is judging or arguing about worth — "They questioned the usefulness of the new form" means they doubted it did any good. A very common phrase is "to outlive its usefulness", said about something old that used to help but no longer does: "That old printer has outlived its usefulness." Usefulness is not a thing you can count, so it has no plural and takes no "a" — say "its usefulness", "the usefulness of", never "a usefulness" or "usefulnesses". It is a somewhat formal, thinking-out-loud sort of word; in ordinary talk people just say "how useful it is". Note that usefulness is about being helpful for a job, not about being valuable in money — an old photo can be precious with almost no usefulness at all.