Trust. A self-signed certificate gives the same encryption.
But I trust a CA. I do not trust you.
So why shouldn't I trust you? Because there's no guarantee that the name on the certificate ("Discount Bob's Hanggliding and BBQ Emporium") was the person who actually created the certificate. I could create a certificate that said "Discount Bob's Hanggliding and BBQ Emporium" and when you go to ritter.vg it'd say "Discount Bob's Hanggliding and BBQ Emporium".
But when I ask a CA to sign my certificate that says "Discount Bob's Hanggliding and BBQ Emporium", they'll ask "Sure, show me some credentials" and I don't have any, so they'll tell me to piss off. But the actual Discount Bob will have those credentials, the CA will sign it. So when you see the certificate, signed by the CA, you'll know that it actually is Discount Bob, because if it weren't the CA wouldn't have signed it.
The purpose of a signed certificate is to verify that the person is actually who he says he. Because the CA said he is, and I trust the CA.
The encryption isn't directly relevant to a certificate - it just gets added in because it's good to have and it goes hand in hand.