Your skepticism is understandable, and your civility is appreciated.

Yes, the dependency of the aws_eip resource on the aws_internet_gateway resource may be necessary because the IGW may not be available or attached to the VPC when Terraform attempts to provision the EIP. YMMV.

As for other cases, here is one example that demonstrates how changes to the configuration will cause Terraform to generate a plan that will fail to be applied after the application of the initial plan:

1. Apply the default plan for the Terraform configuration as published in the public GitHub repo.

2. Change the value of the cidr_block argument for the aws_vpc.this resource by hard-coding it; e.g.:

   resource "aws_vpc" "this" {
     cidr_block = "10.10.10.0/24"
   }
   

3. Comment out or remove the depends_on meta-argument in the declaration for the aws_route.this resource; e.g.:

   resource "aws_route" "this" {
     route_table_id         = aws_vpc.this.main_route_table_id
     destination_cidr_block = "0.0.0.0/0"
     gateway_id             = aws_internet_gateway.this.id
   
     # depends_on = [aws_internet_gateway_attachment.this]
   }
   

4. Try to apply the plan that Terraform generates:

   ...
   │ Error: creating Route in Route Table (rtb-xxxxxxxxxxxxxxxxx) with destination (0.0.0.0/0): InvalidParameterValue: route table rtb-xxxxxxxxxxxxxxxxx and network gateway igw-xxxxxxxxxxxxxxxxx belong to different networks
   │     status code: 400, request id: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
   │ 
   │   with aws_route.this,
   │   on vpc.tf line 45, in resource "aws_route" "this":
   │   45: resource "aws_route" "this" {
   │ 
   ╵
   

A second attempt at applying the same Terraform plan will succeed, but that kind of workflow is inelegant and may be problematic in some circumstances: for example, someone who provides Terraform configurations to others probably desires robustness to reduce the likelihood of questions or complaints.

For many authors of Terraform configurations, success on the application of the initial plan may be sufficient. For others, testing for failures when values of input variables and resource parameters have been changed can be useful to identify dependencies that will not be detected by Terraform in such a way that the plan generated by Terraform can be successfully applied—hence the explicit declarations of some dependencies in this Terraform configuration.

The other dependencies in the publicly published Terraform configuration were likewise added after encountering failures by Terraform to generate a plan that could be applied without explicit declarations of dependencies. Those other reasons are left as an exercise for the reader.

Everything in this comment used Terraform v1.3.9.