Usefulness is how much help something actually is. Prove its usefulness, limited usefulness, the usefulness of the tool.
It is judged by what the thing does rather than by what it promises, which is why the word appears in reviews and assessments. Of a tool or a rule it is measured against a purpose — nothing is useful in the abstract. Note use, useful, and the -ness ending making a quality into a noun.
1nounusefulnessHow much help something actually is.
“I doubt the usefulness of these meetings; we never decide anything.”
usefulness is a quality of helpfulness.
From useful + -ness.
Etymology from Wiktionary, CC BY-SA