See chapter 1.3 of SICP. It talks about the motivation and usage of higher order abstractions in general, and also about fold specifically, which they call 'accumulate'.
I was asking more about code in the wild that uses accumulate instead of the naive recursive formulation. That's typically what we mean by idiomatic.
I see foldl often in haskell, but rarely in scheme. I'm not disagreeing that it's a great idea, just that this is how it 'would have been written'. Personally I find the naive recursive formulation plenty clear.
