It is difficult to understand exactly how much of the profile of a coffee is due to the variety of coffee used simply due to it being incredibly difficult to isolate variety from other aspects of roast or terroir. For example, it is widely accepted that varieties such as SL28, SL34 and Ruiru 11 are exceptionally floral coffees, however given that "floral" will often be a descriptor of Kenyan coffees (where these varieties are most commonly grown) it's tricky to isolate the impact of terroir vs the the impact of variety on producing that flavour note.

Varieties are usually chosen by farmers for economic reasons. You'll see varieties such as Bourbon or Caturra quite often because they're high-yield varieties, thus producing more coffee per hectare than others. Most producers around the world are dependant on coffee for their livelihoods, so picking varieties for yield or pest-resistance tends to trump varieties that might produce a superior cup, but lower yield.