The Library degree exists so I can be remunerated at a salary which I enjoy. 
The MLS degree exists because there is no other way for librarians to distinguish themselves and their work from paraprofessional work. There isn’t a “Librarian Credential” or “Librarian License” exam you can go pass from some government.
As for getting the degree, it’s not that hard to get. At least mine wasn’t. I went to Berkeley’s Library School back in the 1980s and the program was only one year long (2 semesters and summer school). The classes I took then are mostly useless to what I do now, but I could write simple programs in PASCAL and work in dBASE. I don’t think that’s been too useful, but I suppose the background was helpful.
Classes in basic reference skill and introductory cataloging used to be staples of library schools, but they are declining as most schools are pushing the “information science” part of the education.
The problem with this is that most of the library job openings are in public libraries where you really need to know basic reference skills or cataloging (few people do both). You also don’t get taught very well how to manage a library. So, a lot of nuts and bolts problems of libraries are never fixed, e.g. surly employees, ineffective signage, poor communication among cataloging, accesssions, and reference. Another problem you may encounter in using a library is that the librarian you meet at the reference desk may not be the best at being a public library reference librarian, but they may be a great cataloger, but couldn’t find a job doing that. Librarians are not all the same. They usually have some area of specialization, just like attorneys. However, like attorneys, they still know something about all the aspects of the profession.
Another aspect of librarianship you have to learn through the school of hard knocks. That would be how to deal with the public. It’s not easy. If you work in a public library, the operative word is “public”. Anybody can come in and not everyone is easy to deal with. Somedays I think I’ve stepped into the pages of the DSM-IV. A lot of people I help are also very nice and easy to deal with.
In my experience, in public libraries, the professional staff (the librarians) and the paraprofessional staff (the rest of the staff) usually get along fairly well and don’t draw any sharp class lines between themselves. In academic libraries and special libraries (e.g. a private library in a company), there is a greater distinction between the professionals and paraprofessionals.
Anyway, getting back to the OP which I don’t think I answered, I don’t know exactly what you learn in library school. It’s definitely something, but it’s rather hard to quantify and unless you want to subject yourself to the incredible tedium of reading professional library journals, I don’t think you’ll need to know.