A big benefit of making types configurable by closures, rather than hard-coding their behaviors, is that it makes it trivial to add new capabilities. For example, here’s how I'm adding a Splash plugin to my upcoming static site generator using a configurable type:

extension PipelineBuilder {
    mutating func splash() {
        customStep { context in
            context.siteMap.transformItems { item in
                item.body = MarkdownDecorator().decorate(item.body)
            }
        }
    }
}