Verisign, Digicert, someone says that this site is legit. You get a little lock icon and now your bank or shopping cart interaction is more secure.
Self-Signing v using a Certificate Authority and a root certificate and a chain...
If I say my computer is sending what it is sending and that it is legit, but you don't trust me, then who can you trust? Why not a big corporation? And you pay the corporation to say you are who you say you are, and you are done, right?