Let's say you write a Go program using a dependency "github.com/foo/bar" v1.0 and post it on the Internet as a regular file (or multiple files), without using git. Someone else downloads the program and runs the usual Go commands:

    go mod init example.com/program
    go mod tidy
    go build

The go tool will see the "github.com/foo/bar" path in the import statements, download the code from the repository, compile and everything will work.

Now, let's say the "github.com/foo/bar" module gets a backwards-incompatible change, but does not change the import path, and you attempt to do the above process again. This time, the go tool will download the incompatible version of "github.com/foo/bar" and the build will fail - or even worse, succeed but have some logic bugs that will go unnoticed until some massive shit happens.