As I have often said, the librarian may be handicapped by a lack of specialized knowledge. The end user is handicapped by a lack of system knowledge. Often the best result is obtained when they work together. The number of different types of searches is huge. Actually, most are similar or related to previously published papers or books even though the idealized paper in science is absolutely original and thereby different by definition.
Librarians have to deal with a kind of ambivalence on the part of searchers.
Unfortunately, the latter often proves not to be the case, and the intermediary may be the bearer of bad news. Depending upon the searcher's sophistication, it may or may not be surprising that there is an abundance of information available on most subjects. In a college or public library, questions are asked that generally have been asked many times before. Many researchers follow the example of Nobel laureate Peter Medawar who chose to ignore the literature until after the initial phase of exploration was over. In that case, the literature search is done when the resulting paper is about to be submitted for publication.