Similar to the recent question about why we lock our computer, many people send documents as PDF file instead of a doc or html page because it would protect editing the document. And yes, it does add some value because you're making it less easy to do, but it's still easily doable. There are thousands of resources describing how you can edit a PDF, and even official documentation on the PDF format.
So why does everyone, including tech people, think PDF is a much better format to use?
Edit: Thanks for the answers. The weird thing is that people always bring "you can't edit it" as first argument, even at school, and I'm doing application development so it's not like the people there are computer illiterate.
Layout preservation is no issue in my case because we know the teachers use Office 2007 or 2010 on Windows 7, just like us. Still, most people hand documents in as PDFs, and then (when asking) arguing it's so that they can't tamper with what you handed in...