more practical answers
1) Octane under 100. Yeah, for street driving you won't see 100+ octane. Race cars, however, routinely use 100 and higher octane. 100 is also the octane rating for avgas, the most common aviation fuel.
2) Will it hurt your car?
If your car requires regular unleaded (87-88 octane) and you dump 93 in there, yes it will hurt your car. The 93 burns more slowly than 87, resulting in an incomplete burn by the time the exhaust valves open. The unburned fuel is then depositied in the engine as carbon. Eventually, this carbon builds up enough that it stores heat from the burning fuel. THAT causes the next load of fuel to ignite before it's supposed to - it's being ignited by the carbon's heat rather than the spark plug. This is a condition called predetonation, aka ping, and it's very good at tearing up engines very quickly. What's the solution? Higher octane gas. In other words, unless your car is designed for it, the expensive stuff acts like a drug. Your car becomes addicted, and you will eventually not have a choice but to put the expensive stuff in there.