Don't know for sure. But I was guessing it was either "Prove your readership to advertiser", "Security", or "Desire to encourage participation".

The security reason is becoming more common, but yet again, it's the appearance of security. Furnishing an email address does NOTHING to enhance security. Don't think so? Ok, time me... watch how long it takes for me to create a temporary account somewhere, register for your silly board, then never use that address again. And there's no way to link the address back to me.

When a board requires registration to read, I usually move on. Internet's a big place... usually the same content is available elsewhere.