The [Documentation](https://git-scm.com/docs/git-mergetool) does not state why `.orig` files are created by default. Also, their content after the next merge is undefined. Still, there must be some reason for having them created by default.

My best guess is that correcting a failed merge is common and also easier when existing `.orig` files are present (as opposed to repeating the merge) but apparently most users do not know what to do with them.