Librarians do much more than check books in and out of the library. They build the collection and curate it. This requires constantly working to ensure that the collection meets the needs of the school. They weed books that are outdated or no longer meet the needs of the students and staff. The add to the collection. And they solicit input from the school in doing this.
In addition to issuing the "overdue offender lists," they help students find books that match their needs, interests, and preferences. They are adept (or they SHOULD BE) at finding another book "just like this one."
On top of doing inventories at the end of the year, librarians can help kids find the answers to their questions, find resources for their research needs and interests, find reliable sources. There are a series of memes on Facebook attesting to this ability.
In truth, I could spend much more time and space on what it is that librarians can do. Find out for yourself, though, by talking to her or him.