Keep your books from falling over - tab fits under the outermost book and provides support for the rest of the books.
This thing is part of a final project for CPSC 183 Technology and the Law with Prof. Brad Rosen at Yale University. The idea of this project is to ask friends for ideas for physical objects that they would like to have that could be printed on the Makerbot TOM. I have designed them, printed them out and given them to the people who requested them. I also have uploaded them here to Thingiverse to add to the library of physical objects that people find cool, useful, or just plain fun!