Recently, in reading a book on strategy execution, I found a clear test of why an employer should retain you. It simply says:
"How does my work create value for the customer? Does it contribute to my company's value proposition? (If not, or if the answer is unclear or unsatisfactory, the employee should ask whether their job should exist.)"*
I've frequently asked myself the same thing, and it was in this frame of mind that I gave a presentation to our clients. A presentation that included zombies, tight-rope walking, geo-fencing, the Heartbleed vulnerability, and our pants-less Director of IT. It was fun, and my value-add was to keep people awake for that twenty minutes.
*(This came from "Discipline of Market Leaders" by Treacy and Wiersema.)