I agree, I wouldn't want it any other way nowadays, but back then I had to migrate a lot of legacy system to AWS under pressure.
For one part we had a legacy service needing to connect to the services in the ASG and the best way to implement it was with round-robin DNS. So the lambda would update a DNS record contianing all the ASG host ips.
Also, because we had some had some semi stateful legacy instances that where basically lift and shift to AWS, but I wanted to have them in ASG to keep our environment similar until we could refactor them into real cattle.
I don't remember exactly. We did use elb's for all other services. So it was either cost or it had to do with MX record restrictions in that you're not allowed to use CNAMEs in MX records.
For one part we had a legacy service needing to connect to the services in the ASG and the best way to implement it was with round-robin DNS. So the lambda would update a DNS record contianing all the ASG host ips.
Also, because we had some had some semi stateful legacy instances that where basically lift and shift to AWS, but I wanted to have them in ASG to keep our environment similar until we could refactor them into real cattle.