Just this morning a writer sent me a requested full manuscript in a chapter-by-chapter attachment. One email (thank all deities large and small) but containing 17 attached files: one for each chapter.
Of course I wrote back and said: "One file to rule them all by gollum, and don't you forget it my preciousssss."
But I'm deeply perplexed by this strategy. It's akin to sending one chapter per envelope by snail mail.
I can't think of a single reason that you'd ever do this unless the agent specifically requested it. Sending individual chapters means I have to open each file, copy and paste each one into a master document, make sure I got ALL the files, redo the headers and footers, make sure the pagination is correct and then save it. Am I EVER going to do that? Sure: if you're #1 on the New York Times Bestseller list and are my client. And that is IT.
If you've got any insight into why writers do this, I'm interested in hearing it.