You're paying for a service that auto syncs your data across multiple devices efficiently and keeps versioned backups of everything.
The fact that they currently use s3 for storage is an implementation detail.
But this is kind of like saying iOS is just an expensive closed variant of FreeBSD.
Since I'm not drawing a salary from Dropbox's marketing division, I'm not going to go in depth here but I'll just say that Dropbox can be used easily and efficiently by normal people who have never seen a command line. Whereas your home brew python+rsync scripts can only be used by nerds like us.
Maybe you discover the sarcasm in my comment. ;)