For example, the Wikimedia Foundation just published its annual report for 2009/2010. What do you think the PDF file would be called — "wikimedia_report_20092010.pdf" or so, perhaps?
Not quite; it's actually called "AR_web_all-spreads_24mar11_72_FINAL.pdf". This'll tell a Wikimedia employee exactly what the file contains, I imagine, especially if they're working on the report (annual report, web version, all spreads (?), 2011-03-24, 72 dpi, final version), but someone who downloaded the file and then comes across it on their hard drive a year later will have absolutely no idea what might lurk inside until they actually open it.
Yes, I know, files can be renamed. But still — please, use informative file names. :)