Even after the rumors make it onto the Mac web, doubters and believers still argue back and forth over the validity of the photos or reports. I've read a few articles that grumble about this state of affairs. Rumors, especially lately, generate a lot of commentary and discussion. Content is what drives web sites, and rumors provide writers with plenty of content to fill their pages. Whether the content is always good is another matter.
Secrecy guarantees lively discussion before and after the product is introduced.
The second benefit is a spin-off of the first. All this discussion creates a buzz in the Mac and computer community. We get excited, we talk about it, and we want to see which rumors are true and which are not. This buzz is not restricted to Mac-centric web sites. Slashdot and The Register are two sites that occasionally post Mac rumors.