Marius Storm-Olsen [off-list ref] writes:

2) Lets you replace both name and email of an author/committer, based
on a name and/or email. So, should you have done commits with faulty
address, or if an old email simply isn't valid anymore, you can add
a mapping for that to replace it. So, the old style mapping is
    Proper Name [off-list ref]

while this patch series adds support for
    Proper Name [off-list ref] [off-list ref]
    Proper Name [off-list ref] Commit Name [off-list ref]

This extended mapping is necessary when a company wants to have their
repositories open to the public, but needs to protect the identities
of the developers. It enables you to only show nicks and standardized
emails, like 'Dev123 [off-list ref]' in the public repo, but by
using an private mailmap file, map the name back to
'John Doe [off-list ref]' inside the company.

I do not find the "necessary" argument very convincing nor I find the
particular use case sane.  You may want to do things that way, but I do
not know if it is the best way to go about it.

The new mapping however brings in more flexibility, and there may be other
use cases where people benefit from that flexibility.  I am slightly in
favor than neutral to this new feature.