But in many countries 100LL is quite rare e.g. Italy.
TOTAL tried to split the market here with 91UL, even giving away free bowsers, but failed because a) the price was almost the same and b) airports didn't want to halve their 100LL purchases and pay more and become uncompetitive (the GA business here is highly competitive on fuel costs and even food costs).
If - like most - you never leave your one country, then you may use 91UL all the time. But internationally it is rare.
My TB20 (IO540-C4D5D 8.5:1 CR) I can use 91UL or higher.