Instances can have many secondary interfaces
Use case: If some software is allocated per macaddress, then you can relocate that license by moving ENI to other EC2 Instance
Use case 2: two interfaces per each type of traffic, instance connected to two subnets.
Use case 3: Different security groups for each ENI, if needed, it’s available
OS does not see Public IP4
Within VPC, Public DNS corresponding to the Public IP resolves to Private IP in public, so in case if you have 2 instances with public IP, they won’t talk to each other over internet, but within VPC.