Yes, it will be to accommodate people not wanting to work every Saturday. When someone only works every other Saturday, that means every other week they work a weekday instead, which means the person who usually works that weekday on the 'other' week needs to be scheduled somewhere else to accommodate and so it goes on!
I am a Saturdays-only library assistant working in a library where we always have three people on shift, so the five people I work with rotate in some complicated pattern I can't understand!
I really enjoy my job, but I have to be honest and say part of the reason I like it so much is that I only do it for 6 hours a week....I think if I did it full time I would find it somewhat boring/repetitive (however, with full-time work I can see there would be opportunities to expand the role, e.g. be responsible for volunteer training and placement, or building health and safety, or book donations across the district (yes, our service takes in donations)).
On the plus side the job is entirely stress-free and you don't think about it once you've locked up; having come from a corporate world of clients and deadlines, the library service is absolute heaven! It's a very pleasant working environment, its fun to chat to the customers and volunteers, the staff are all lovely and interesting (and over-qualified!) and its wonderful to have access to so many books (top tip: use your mobile to photograph the cover of any books that appeal whenever you come across them [which is usually when they are on the way to another branch!]). It's also nice to feel you "make a difference" e.g. helping someone on the computers, finding books on eating disorders for a teenager, or simply helping an elderly person use the lift. It's a rewarding role.
There is room for progression in my council - from library assistant (me) to library officer (my boss - back office; looking after a team of assistants and the libraries more generally) and on upwards, although for me the officer job does not appeal (not least because they are the ones who have to do the timetabling! :)).