It's useful for more general folds over more general types. If you have a tree structure then you might not want to fold from the left or the right but instead

It's useful for more general folds over more general types. If you have a tree structure then you might not want to fold from the left or the right but instead in multiple places in parallel and then combine them at the end * + (* + (* + (* + (* + (* + *))))) versus (((((* + *) + *) + *) + *) + *) + * versus ((* + *) + *) + (* + (* + *))