More specifically, it makes it so that Apple is not forced to conduct mass surveillance by giving up everything when they receive a legal wiretap order, in the vein of Lavabit.
If you see someone else running a message system that has no way for the cops to read it, that should be a sign that it's insecure -- not technologically, but architecturally.
Ex: Email, which is run by any number of providers, however if an email client is configured to use PGP and access is via POP/IMAP and not webmail, it's still secure as far as we know. A message system that may not be email, but still doesn't store keys on the server, still provides no way for cops to read it. Except perhaps to see some message was sent, not what the message was.
What's that supposed to mean? What about OTR or TextSecure or PGP over email?