As developers, we all know how useful and convenient variables are - define something once and use it everywhere. We have extended this idea with the online security testing suite. Apart from the organizational benefits, you have a single source of truth which allows for quicker edits.
The beauty of variables is that they are suitable for use in the simplest of cases to the most complex ones. The things you can keep inside a variable vary from single word strings to text builders containing lists with thousands of elements. It is up to you to decide what's worth putting in.