ECS has great integration with other AWS services like IAM, Cloudwatch, Autoscaling, Application load balancer, S3, EBS volumes. So if you are already using AWS then ECS will come in handy.
There are other option of ECS also available like EKS, DC/OS or Docker swarm.
Without ECS it gets messy to manage how containers are deployed, where they get deployed, auto service discovery, auto error detection and remediation, load balancing and other stuff.