Git Commit Spoofing is where a person uses the Git identity of another person to modify commits pushed to GitHub, assuming they can obtain write permission to a branch or via a PR.
Signing is especially useful for those contributing changes via a pull request. Signing provides greater confidence that the contribution is from a verifyable account.
Working in the Financial sector for many years, signing of commits has become manditory to reduce risk and increase traceability for audit trails.
All Practicalli commits are now signed using a passphrase protected SSH key, registered as a signing key with the GitHub account. All contributions via pull request should be also be signed, either with GPG or SSH keys.