My ironhead loves high octane fuel (keep in mind it's 35 years old). When I ran 100LL aviation fuel in it, it was much smoother and ran better. When I shut it down it would stop immediately with no dieseling like a Ford Pinto. There was also no knock.
Which if your IH would knock with the lower octane fuels is exactly why the higher octane stuff made it run so well. The higher the octane, the better it fights knock.
You see it out there in 'berdoo because it is on the way to the river and desert. The guys with boats and off-road vehicles really trick them out and the engines in those vehicles, not needing to worry about smog rules, are tuned for high performance. They run blowers, turbos, and high compression engines with lots of timing all of which will increase an engine's dendency to knock. The 100 octane race fuel will combat that. As others here have said, there is no other benefit to running higher octane fuel if you're not getting knock now with the 91 or less.
As already stated, it's more of a waste of money for a street motor...There is also an increase in heat during street riding with greater than 100-octane and it can lean the air/fuel mixture out some too!!!
Any of the newer twinky bikes don't waste your time . Older high compression shovels or sporties is a whole different ballgame they like higher octane gas , I run 11-1 comp in my shovel a mix at around 96 or 98 oct. wakes the old girl rather nicely .