I don't have a technical background, so I'm presuming there's a simple answer for this:
Why does Gravatar require you to create a hash of an email address before sending a request to their system? Is there a technical (or social) reason not to just use the email address?
It's to prevent exposing the email addresses to harvesting. If the raw email addresses were used in the avatar URL, it would be a simple task for spammers to harvest those email addresses for their nefarious purposes by scraping the HTML/DOM of any forum using Gravatar.