I think it is a valid question and difficult to get answers.
note I am a newbie too in this area and have limited knowledge, never mind I have enjoyed working with packer and will point some of the points.
First, I would say you want to compare packer again cobbler (+ koan) or other tools in this area (Foreman, Katello ...) as both (if not all but I dont have enough knowledge about all those tools) are using the kickstarter format to create machines;
Second, packer is nice in a way that you can create machines (running any kind of OS from debian, fedora or windows family - I use it a lot for centos and ubuntu machines) but works great for cloud containers too (AWS, digital ocean and many others)
Third, packer works with templates where you define builders
(generate a machine images) so from the same template you can generate machine for virtualbox/vmware or the cloud and they will have the same setup. what is really nice now for new comers is that you can find templates to generate almost all kind of machine image (all flavors of linux, windows ...) so it is easier to start
Finally, its a matter of taste. I came to packer from using vagrant and both are working just great together.