The Arbitrary Settings module is intended for programmers. In of itself it's not useful. The module adds a section at the bottom of the Settings -> Main tab, where you can define you own settings for storage in the Form Tools database.
What do those settings contain and do? Anything you want. Programmers can then choose to reference those values in their code.
It just provides a simple interface to allow storage of arbitrary data that's controlled by the Form Tools administrator.