A) It has to do with the combustion temperature and ability of the fuel, and of course will mainly affect the combustion chamber, intake and exhaust, and the surrounding components.
B) More important, the additives of regular gas and the chemical composition will create problems in the fuel system, from the tank, pump, sensors, pipes and valves to the collector tubes and injectors... The injectors will dirty up abnormally faster from such gunk and require either thorough clean up or replacement more often.
I've done a bit of research and misc. reading on the subject through the years, and everything said to use the lowest test gas that doesn't make your engine knock (ping) under load. The variable timing in our (and other modern) engines eliminates the knocking but should lower the mpg with lower octane ratings. So, if you keep track of your mileage you can see "what's right for you".
Back in the day, I had a '57 Pontiac with a 11:1 comp ratio that demanded Golden ESSO or Gulf Crest which had octane ratings around 104. I could set the timing back just a hair and it would run fine on normal hi-test, maybe with a little less power, but the sacrifice in mileage wasn't worth it.
As far as additives go, they may be different in different brands, but none want to sell gas that messes up engines. When they were using lead tetraethyl, they just put more in hi-test than in regular, so I guess they still do that with whatever the substitute is.
Personally, I'll go to the 91 if I find the no- meth 89 that I can get lowers the mpg.