My career interests are not in academia, and yet I'm a postdoc at a university. This is certainly a plot twist in my life that I could not have predicted 3 years ago when I decided that academia was not the life for me.
Why am I here then? One of my mentors, Tepring Piquado, often referred to her experience as a postdoc as grad school plus. This always felt reasonable since postdocs are technically training positions but I don't feel it fully made sense until I was faced with doing a postdoc.
My postdoc is a bit of a field switch. My PhD was in developing theoretical understanding for thermal density functional theory using simple models and now I'm working in applying ground-state density functional theory to real (and comparatively complicated!) systems. This is a key aspect to grad school plus. I spent 5 years becoming an expert in one thing and now I'm expected to become competent and productive in something entirely new within a much shorter span of time. This is powerful preparation for the real-world and is a type of preparation that just doing grad school doesn't prepare you for.
During grad school it is easy to lose sight of the big picture especially in terms of what you know and don't know. I made it a point during my PhD to learn skills beyond the scope of my research, such as science communication, but as I neared the end I painfully noticed that I was lacking a skill expected of most theoretical physicists. My research group was old school in that we did lots of pencil and paper theory and simple calculations whereas most theorists spend a lot of time coding. So, grad school plus is a perfect time to fill the holes left by grad school -- in my case I've spent the past month learning Python and neural networks. By time my postdoc ends I intend to have a skill set that allows me to enter data science if I so choose.
The over riding theme: it is easy to burrow into a niche and a postdoc is a golden opportunity to expand your skills so that you're more competitive on the job market. A PhD is a powerful thing (or so they tell me) and having diverse skills to back it up is even better. I've always fancied myself more of a jack of all trades and I'm enjoying this opportunity to move back towards that role.