If the task is sent for approval to the Project Group is sufficient for one person belonging to that group to approve that task to change status to Approved.
Also, with Project Group, you can control who can be added as an approver for that task to a select number of people by making the Project Group not editable by Project Owner.
I am sure there are a few other consideration that other members might want to add.
Agreed. Project Groups are kinda like Local Variables. They are really good for making sure you don't "hardcode" to a single person. For example, in a template, if you set up a PERSON as an approver, that means you need to make a new version of the template to change that person. But if you use a Project Group (populated with a Global Group), you can modify that setup "remotely" (Admin).