What's the use case for committing both the freshly created file and it's exclusion in .gitignore?

Hi there!

I hope that the subject of my message (i.e. the question) is exhaustive enough, so I'll just stick to reproducing my issue.

Steps to reproduce:

1. Create a new file.
2. Stage the file.
3. Add the file to .gitignore.
4. Stage the .gitignore.
5. Commit changes.

I imagined that the file would now be removed from the stage (because it's ignored now and not yet committed) but it isn't. Where this behavior would be desirable? I know that a 'git add' command can be invoked with an '-f' flag, which would yield the same result, but I can't come up with an explanation where can it be applied.