Once you've used aws configure, you will need to manually remove the credentials and config folders under ~/.aws. If you do not do this, any roles that you assign will be ignored, as the assigned credentials are overriding.
Roles allow you to not use Access Key ID's and Secret Access Keys
Roles are preferred from a security perspective
Roles are controlled by policies
You can change a policy on a role and it will take immediate affect
You can attach and detach roles to running EC2 instances without having to stop or terminate these instances