I am confused. Does Spinnaker use HashiCorp's Packer for baking or is it another tool? The reason for asking this that a folder called packer resides in the github repo of rosco https://github.com/spinnaker/rosco/blob/master/rosco-web/config/packer/docker.json
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.