Most libraries have their entire collection of books, microfilm, CD-ROMs, video, and audio on a computerized catalog, which needs to be maintained and often have the software customized. In addition, compatibility between all the information sources and catalog databases, and the computer hardware and software for client use, needs to be updated regularly. In addition, many subscriptions to journals and other information sources are online, and require libraries to maintain secure access to prevent unauthorized access by non-paying users (i.e. users not formally affiliated with the institution), and many universities make these online sources available through student and faculty accounts on the university's main computer network, requiring maintenance of system of controlled access from remote locations.
A quick Google search pulled up the following required courses for an MS in Library Science from the Univ. of Kentucky:
"The required core courses are LIS600 Information in Society, LIS 601 Information Sources and Services, LIS 602 Information Storage and Retrieval, and LIS 603 Management in Library and Information Science. Qualifying technology courses are LIS 636 Microcomputers in Libraries and Information Centers, LIS 637 Information Technology, LIS 638 Internet Technologies and Information Services, and LIS 668 Information Systems Design."
While this is obviously not the equivalent of an M.S. in Computer Science, neither is it equivalent to an M.A. in English or History. Any many people who get these degrees do a lot more technology coursework than the basic requirements, in order to qualify for better jobs in large specialized libraries.
The average circulation desk clerk at a little public library doesn't have or need a degree in library science. However, university and professional libraries (e.g. law firms private libraries) do require these specially qualified professionals.