Does Spinnaker's rosco use HashiCorp's packer or is it another tool?
From spinnaker own documentation


  Rosco is the bakery. It is used to produce machine images (e.g. GCE
  images, AWS AMIs, Azure VM images). It presently wraps packer, but
  will shortly be expanded to support additional mechanisms for
  producing images.


So yes, rosco uses HashiCorp's packer.

The main reason to use packer is that you just need to change the provider used to create a docker image or an AWS ami or an Azure image and keep your building step identical.