It is her job to watch over the collection. I think the task is similar to caring for other collections of valuable objects, at least in some ways. But this collection of books and movies and pamphlets and software has the extra feature of loaning. The collection gets handled, processed, kept, loaned out, returned and generally tracked. So, the items age, wear out, become outdated.
A garden needs to be tended, watered, weeded. So, with a collection of books. They, too, can be worn out from handling, opening, closing, dropped. They can become out of date: the World Almanac for 1937 might not be of interest to anyone anymore.
The librarian is responsible for having as up-to-date and valuable a collection as possible. It is valuable to the extent it helps. Gives information, inspires, corrects, extends thinking, creates understanding, launches dreams. Have you heard of The Boy Who Harnessed the Wind by Kankwamba, Mealer, and Hymas? A village in the African country of Malawi had no electricity and a very severe drought. William Kankwamba read in a school library about electricity and used the knowledge gained to create electricity with a windmill.
Both of our local libraries have shelves showing new books. The other day, I found "Approaching the End of Life" by Donna Schaper, a minister for 40 years, and "8 Keys to Practicing Mindfulness" by Manuela Mischke-Reeds. They had been sitting on my shelf and the time of the loan was ticking by. I sat down with both of them this morning and in about 3 minutes per book, found they were both likely to be of value. Thanks to the librarian's eye and judgment, I found out about these books. Without his continual reading about what is being printed, I would not have. He told me that he uses "Booklist", "Publisher's Weekly" and "Library Journal" to make decisions about spending a tight budget for the benefit of the town's readers. These days, any library may actually be an outpost for a moving collection. We can look at what is available in multiple libraries and request that an item we want, be trucked here to be borrowed.