One use-case: at my last gig, we assigned each customer a new sub-domain, under our main one. Route 53 worked great for this. We used boto (python API for AWS) to create EC2 instances, get a static IP, and then create a new DNS entry in Route 53 to map to that IP, all in about 5 minutes.
Prior to Route 53, we used to do the mapping manually, as our primary DNS provider did not have an API.
Prior to Route 53, we used to do the mapping manually, as our primary DNS provider did not have an API.