R/haskell - Was simplified subsumption worth it for industry Haskell

[atravers]

This seems relevant:

Useful applications of partial application

I am interested in finding useful applications of partial application. I’m pretty sure I understand partial application but what I’d like to be able to do is say to a student (or indeed a teacher - this is at the UK’s sixth form level) that this is one use of it, and what its advantages are. Is there an efficiency advantage? Given a function f a b, if a is costly to execute, then does defining g = f a then mean that g b is more efficient? I can see some benefits in generating specific ‘version…

Why is the programmer being asked to do what looks to me like a job for the implementation (eta-expanding partial applications and parameters which are functions)?