I've needed to do this for a bunch of reasons.

1. Sometimes I do a scraping project where I want the commit dates to match when the data actually changed, reconstructed from sources like the Internet Archive or the dates on https://platform.claude.com/docs/en/release-notes/system-pro... - for example here: https://github.com/simonw/research/commits/main/extract-syst... - or this project that attempted to reconstruct the history of Tim Berners-Lee's original browser: https://github.com/simonw/1991-WWW-NeXT-Implementation/commi...

2. Sometimes I split a Git repo into two, and I want to preserve the commit history (authors and dates) on the files that end up in the new repo - effectively replay the history of just one folder from the original to help create the new one.

3. I occasionally mess up cleaning up and merging a PR from an open source contributor such that the work is incorrectly credited to me. I'll fix the commit so that it credits the right person.