The question is poor because it specifically gives such a simple example of currying. You can't understand certain concepts without understanding when they are used, and when they aren't.

If the OP had instead said "I don't understand why List.foldl is curried instead of just accepting a tuple." Then you'd see a more productive discussion. Perhaps touching on the differences between functional language implementations.

As it is, since the question isn't formulated well, the answers are all over the place.