When your bike doesn’t ride properly you can feel it. It makes a clicking sound, there’s something rubbing, the tires need air, or it’s harder to pedal than it should be. You can try figuring out what’s wrong for yourself but if you don’t have bike accessories it won’t much matter.
Even if you do, you can find yourself in the same predicament after a week of heavy riding. If you carry bike accessories with you, however, you can protect against problems while you ride, which will allow you to get home or to a bike shop safely.
I carry a wrench (in part because my rear wheel isn’t quick release) because if anything loosens up or something’s wrong with my derailleur I can adjust it, which will help at least for a little while. I carry a spare tube and a patch kit, just in case, as well as a butter knife which is great for popping off the tire from the rim. I also carry some bike lube, which goes a long way toward a smoother ride, especially after being caught in the rain. These bike supplies aren’t too heavy and I can’t tell you how many times I’ve used them to save my ride.