I'm not sure if the OP meant 10" overall, or a knife with a 10" blade.
For many years I carried a fixed-blade with a 5" blade (overall length about 10+") about everywhere I went. And my reasons for carrying it weren't just based on what I planned to use it for, they were also motivated by any unexpected uses that might present themselves. I mostly used it at various jobs over the years (shipping dock, construction), but once in a while some unexpected uses would arise. Here's on of my more interesting stories (I've told it here before years ago).
I was out on a ride in the middle of nowhere (motorcycle), and I stopped at a little gas station/convenience store. As I'm sitting in the parking lot I hear this horrible racket. A car is coming down the street, and the plastic cowling under the front fender had come unattached at one side and was dragging under the car.
The female driver pulled into the gas station parking lot, and was in a panic. She had hit a big rock n the road, which had broken the cowling loose, and she had no idea what to do about it. This wasn't the kind of gas station with a mechanic, so she was out of luck there.
There was no one else around to help her, so I offered my assistance. After inspecting the problem, I came to the conclusion that the best course of action was to remove the cowling, and the woman agreed. Easier said than done. Although it was plastic, it was tough plastic, and it was riveted to the car. I tried to pull it off, no go. So after asking the woman's permission, I pulled out my knife (Wilson Tactical model 25) and I proceeded to pry the cowling off, popping it loose one rivet at a time, requiring some significant effort. I don't think many small fb's, or folders, could have handled the task.
The mangled cowling removed, I tosses it in a dumpster. The woman thanked me and we went our separate ways.
In my current life, I no longer feel that openly carrying a fixed-blade is practical for me. But as far as the question "Why carry ...", well, as long as its legal, and as long as it fits in with your life and lifestyle, why not. You never know what unexpected uses will present themselves.
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