An elastic network interface is defined as a virtual network interface https://docs.aws.amazon.com/vpc/latest/userguide/VPC_ElasticNetworkInterfaces.html.
I was wondering what use cases exist of this. Why would not I simply put directly an instance instead of having to use the eni, and attach it to an instance.
Thks!
Decoupling the ENI from the EC2 instance allows you far more flexibility.
You can connect several ENIs to the same EC2 instance and be attached to different subnets, for example:
ENI A would be connected to a public-facing subnet
ENI B would connected to an internal pivate subnet
You could, for example, attach an ENI to a running EC2 instance, or you could have it live after the EC2 instance is deleted.
As a form of "cheap" high availability: Attach a ENI to an EC2 instance; if the instance dies launch another one and attach the ENI to it. Traffic flow will only be interrupted for a short period.
There are many scenarios where this flexibility comes in handy.