Sometimes, you don't want your personal email associated with a work project, or your work email associated with a personal project. You can leverage git identities to ensure this separation. Bonus: cryptographically sign your commits.

The keys have a User ID packet with a userId field that contains a name and email address, which is an identity. I prefer to keep my identities separate from one another. For me it's not necessarily about security, it's more so about compartmentalization.

It's in the second sentence of the post =P

"This is a procedure for leveraging git aliases to set an identity at the project level..."

Generate a new key and update the global git config with the new public key hash. Then add the new public key to Github/Gitlab to support verification of signed commits with new key.

But that would invalidate the old signed commits, wouldn't it?

Pretty sure you can keep your old keys in there. You just don't use the old key for signing.