The main differences are price points and restrictions on fares.
QF's "simpler" categories are reasonable in this respect - ie Flex v Discount - most things within borad categories are the same, or similiar(eg a cxl fee may be go up the more discounted a ticket) but in general you're looking at price points.
Eg QF flex fares book into, usually Y B M H K buckets iirc - Y being the absolute full fare, and K being the cheapest price point. Most fare types in these ranges will be more or less the same though.
QF's logic is you don't need to know if it's a K fare or a Y fare because "flex" therefore they're making it simpler. To a degree. I disagree, but I am one who likes all the details
And of course important for partner earn, specially for more discounted fares.