Amazon recently launched the ability to upload your own ssh public key to EC2 so that it can be passed to new instances when they are launched. Prior to this you always had to use an ssh keypair that was generated by Amazon.
Ok, so it’s not recent any more, but it still seems pretty unknown. I use PhpStorm for my IDE and needed to have a private/public key in order to connect via SFTP. With this change I should be able to connect now!