An argument may warrant additional support in the form of additional evidence when
a) you need to make your essay longer
b) your audience strongly agrees with your argument
c) your argument is supported by expert opinion
d) none of the above

  this riddles' answer is library.

  the "leaves" are a reference to the books' pages that the library contains; the branches are a network of libraries that connect multiple libraries' collections, resources, and items in a system of a common database, integrating them; the branches' organization are a useful management tool for coordinating the multiple collections many libraries have.