I'm a fan of Cafe Racer styling and design principles for my Triumph Bonneville.
That means: simplicity, reduced weight, and "period" styling".
With that in mind, I was loathe to add any kind of luggage to my motorcycle - if I needed to transport anything, I would wear a backpack or try to fit it in a cargo net on the back of my seat. When I would ride with friends for the day, I'd sometimes bring a small tank bag to hold water, an external battery pack and cable for my iPhone, and some snacks. That worked fine for the first couple of years.
Now that I live in Austin, Texas, I ride my bike all year long and as often as I can. At times, I find myself in a weird position: I want to bring an item with me, like a notebook, but don't want to throw on a bulky, sweat-inducing backpack for something that weighs just a few ounces, but is too big to fit in my inside jacket pocket. (Yes, I'm OCD. Why do you ask?) 😉
So, I considered [panniers]. One would suffice, as I could always supplement my total carrying capacity with a backpack if I found myself in a position where I had a bunch of stuff to carry but didn't want to drive my car.
I started combing the interwebs, looking for a pannier that had sufficient storage for my MacBook Pro, notebook and maybe some clothing layers for long riding days when the temperature might swing widely or gets wet.