Lots of non-git tools use them. Some that come to mind are ripgrep (by default ignores .gitignore'd files), some editors (greying them out in the file selector, not searching them, etc), rustic (not backing them up), probably more.

For one thing, many AI code helpers use the contents of .gitignore as a suggestion for files to not include in their context or changes.

My bad, I misinterpreted the proposal.