Why use Dropbox when you can already build such a system yourself quite trivially by getting an FTP account, mounting it locally with curlftpfs, and then using SVN or CVS on the mounted filesystem, back in 2007 even?
Because I can't store my gmail and Google Docs documents on S3, and because Google Photos is easier to use than cramming my stuff to S3.
Yup. It's not just that I use Docs, but I don't know what I'd do without Google Drive Search across all my documents. Same with searching photos using text search.
Also Google Drive for Desktop. I'm sure there's an equivalent for S3 but I don't know if it works as well.