I have seen and experienced some very bad burns that occurred when someone shorted out the main battery voltage with a wrench and then couldn't let go of the wrench fast enough. Even if you aren't working anywhere near the battery, the wiring stretches from bumper to bumper. All you have to do is short circuit a hot wire to ground and it can get red-hot in a fraction of a second. This can start a fire or just give you a nasty burn on your little pinkie, not to mention cause the car to lurch forward or backward if you happen to short the starter wires and the car is in gear. The car is not likely to start accidentally if the battery is disconnected.
Batteries can explode.
I didn't believe this until one blew up while I was connecting jumper cables to it. It made a very loud noise, and splattered battery acid all over the place, not to mention flinging jagged bits of plastic in all directions at very high velocity. I think many injuries could be prevented if people always disconnected the battery cables before working on the car. If you are trying to diagnose an electrical problem you will have to leave the battery connected, be extra careful. Some parts of the car get very hot, and stay hot for quite a while after the engine is shut off.