With a doughnut or run-flats, you have to limp to a tire shop to repair the tire or get a new one, and if you don't have a spare at all, it's a serious inconvenience at best, and a crisis at worst.
That's the thing: I do get a punctured tire... sometimes. Maybe every 2-3 years. Not critical, just losing pressure. Not urgent. Last time, local shop fixed it for $28. They gave me a little freezer bag with the nail that they pulled out of the tire.
Now, did I ever have a flat tire that needs a replacement like right now, with the spare tire? Well, I don't remember having that. In several decades of driving.
For the record, I do go to service to get regular maintenance, so every like 3-4 years or so I change tires. I guess that helps.
I'm not carrying it for if I get a slow leak on my work commute, I'm carrying it for when I'm taking forest roads in durango, or driving across nebraska at 2am. I carry a blanket and chains in winter for the same reasons.