What are the main use cases for enterprise search?
The main use cases for enterprise search are customer support answers, engineering context and decision history, HR and policy questions, sales collateral and account history, legal obligation lookup, and IT onboarding. All six share one property: the answer exists in writing somewhere, but not in one place, so it is faster for an employee to interrupt a colleague than to search for it.
That last sentence is the actual measurement. If people in your company ask each other questions that are already documented, you have an enterprise search use case, and its size is the number of those interruptions per week multiplied by two people's time. Companies usually discover the volume is much larger than they assumed, because interruptions in chat feel free and never appear in a report.
A useful way to sort candidate use cases is by whether the question is stable or situational. Stable questions (parental leave, expense limits, the security questionnaire) repeat with the same answer and can partly be solved by better documentation. Situational questions (why this customer is angry, why this service behaves this way) are different every time and can only be solved by search, because you cannot pre-write an answer to a question nobody has asked yet.