After writing my essay, I prepend the introduction.
I write an essay plan and develop my arguments based on the relevant research. Near the end, I have to prune it down to fit the word count, which sometimes means I might have to cut out the least important points. My penultimate paragraph is the summary. My introduction comes last because it is only then that I know for sure what the essay says.
Similarly, an editor might prepend a foreword to an anthology, or an author might prepend a dedication of her novel.