Average computer data is binary, like a program or a photo or an MP3. When you want to deliver this through a computer network, you either need a protocol which is transparent like HTTP, or you have to use some funny encoding like base64.
Knowing length in advance is generally good, see Framing.
coyo: psyc will soon support in-band unaltered binary packets coyo: which xmpp can only dream of having coyo: and irc would just stare blankly at you for asking
Don't laugh, but PSYC isn't semantically all that distant from what the W3C experts expect from a potential future binary XML format. Binary XML and the parallels to PSYC are discussed on the Syntax page.