street vehicles such as your daily driven cars dont need 93 or hi octane gas. Like 600dubb said its the flash point of the gas. The higher the octane rating the higher the temp it needs to ignite. Now lets say u have a 60s muscle car no technology in the ignition system but you have a modded engine. You will want to use high octaine becuase your gas has such a long way to travel before it gets to the igntion chamber(carb, intake runners, ignition chamber). And since the engine is modded your motor is running hotter. In this case you do not want your gas "pre igniting" before it gets to the compression stroke which is called knock. This is bad. But since on your bikes and daily driver cars, your efi and computer prevent this from happening. If your computer senses high temps or something that might cuase knock it will do things to prevent it such as retard timing etc. So running 93 in your toyota camary wont make it last any longer and your just wasting your hard earned money. The only time i would consider running high octane would be if your ride is modded where your fuel map/timing is custom, or you have a moderatly to highly modded carburated application. Other then that save your money for mods. hope that clears things up! 
cedric
somewhere in all that wrong info there is a little piece of fact.....
I still cant figure out why so many people have such a hard time understanding what the octain rating is, Octaine is resistance to detination, nothing more nothing less, the number you see at the pump is the average of the research and motor, rating for the fuel you are getting. Research rating is determined with some math and some one in a lab coat, motor rating is actually determined in a running engine with a moveable cylinder, that is moved closer to the crank untill detonation is sensed. By moving the cylinder closer to the crankshaft, the compression ratio is increased.
There are 2 major factors that determine how much octain you need, compression and timing, the higher compression, or more timing advance you have the more octaine you need to prevent deto (and the more power you should be able to make).
Of your engine is set up to be safe on 86, you wont gain anything but an lighter wallet by running higher, and many engines make less power on fuel with too high of a rating.
UNLESS it is an efi knock sensor equiped engine with agressive programming, that always runs on the edge of deto.
as for the muscle car argument, A normal all cast iron V-8 can only run about 9:1 compression on todays pump gas,(aluminum heads knock sensors and efi have allowed this to be pushed to about 10:1) but back in the 60's you could buy a car running 11:1, and have no problems, because of the high octaine, leaded fuel that was for sale at every corner.
