Why are "certificates" needed, if the GNU Privacy Guards already supports signing?

[Martin Vahi]

Could someone please explain, why is there
another layer of containers used for signing
outgoing e-mail, generally called as "certificates", if
e-mail can be signed by using the GNU Privacy Guard and
my secret key?

This question emanates from the fact that
the key manager at Thunderbird-Enigmail is
capable of using the system GNU Privacy guard
with my set of keys for decryption, but
when I changed my key-pair due to the old one
expiring, I ran to the nasty issue of
figuring out, how to create a "certificate"
to just sign my outgoing e-mails with the new
key. For the old key I somehow figured it out,
but it was years ago, but I remember that
I was puzzled the same way back then, like,

WHY A HELL DO I NEED TO GENERATE ANOTHER
CONTAINER, a "certificate", IF I ALREADY
HAVE THE GNU Privacy Guard private key and
the Enigmail interfaces with the GNU Privacy Guard just fine?

If that extra container is necessary due to
some legacy reasons, dumb standards or something,
then couldn't the Enigmail just automatically
use the GNU Privacy Guard key pair and
generate that wrapping container, the "certificate",
and use it transparently without making the
end users figure out, how to manually do
some file conversions?

May be I have totally misunderstood something,
but this is my current (2016_02) understanding
of the issue.

Thank You for reading this letter,
I hope to be thoroughly mistaken. :-D

[Rob]

Could someone please explain, why is there
another layer of containers used for signing
outgoing e-mail, generally called as "certificates", if
e-mail can be signed by using the GNU Privacy Guard and
my secret key?

Sure. You're getting confused by terminology. Most people talk about
their public key or private key -- but that's incorrect language:
they're really talking about their public or private certificates.
(Their keys are just small bits of data embedded in their certificates.)

You seem to think certificates are something new. They're not. You've
been using them all this time. What you call your 'keypair' is really
your certificate.

Look through the official GnuPG FAQ and see how often it uses the term
"key" and how it uses it -- then do the same for "certificate".

https://www.gnupg.org/faq/gnupg-faq.html#define_key
https://www.gnupg.org/faq/gnupg-faq.html#define_certificate

WHY A HELL DO I NEED TO GENERATE ANOTHER
CONTAINER, a "certificate", IF I ALREADY
HAVE THE GNU Privacy Guard private key and
the Enigmail interfaces with the GNU Privacy Guard just fine?

If you have a certificate you've generated with GnuPG, Enigmail can use it.