Chain tool and quick link will get you going in a number of low probability situations.
If your chain is proper length, then removing one damaged link will make the big/big combination unusable, but otherwise you're good.
If your rear derailleur fails, the same items could get your rig rolling as a single-speed.
I had a quick-link come apart on a ride, in the midst of a rough shift. I was halfway through a brevet, solo, in rural Ohio. I found one half of the quick-link pretty quickly, but never found the second. My spare quick-link saved my ride; I was rolling again in 10 minutes.
If you look in the repair kit of cyclotourists and randonneurs, it often tells the stories of their experiences. I've encountered some oddities like cleats and once a left crank-arm, but I think chain tools are next in line after flat repair and spokes. I carry two quick-links and a multi-tool with built in chain-break.