It’s no different than having to know the associativity of an operator. Does “a - b - c” mean “(a - b) - c” or “a - (b - c)”? The former, for no reason other than a long history of convention. Same goes for function application. And you could make the same argument about languages that require parentheses for function calls: what does “a(b)(c)” mean? “(a(b))(c)” or “a((b)(c))”?

What’s gained is that the syntax mirrors the semantics. If you have a function “f : int -> bool -> int”, then “f 5” has type “bool -> int” and “f 5 true” has type “int”. It’s not like in Forth where you can’t tell from the syntax how many arguments a function takes.

That said, I do think right-associative function application may be more intuitive—I like having it in Perl, for example.