It's a stress test for the servers. It says in the e-mail that if you participate for at least an hour, it'll help you get into the "real" beta testing.
Also, you get a sweet monkey!
Edit: Although I question their timing. It starts at 6pm Eastern (Friday) which is workable for me in the Central zone but would suck for someone on the west coast who gets home around 5ish their time. They'd have maybe an hour to get in, assuming no problems.
Edited, Sep 4th 2013 12:28pm by Jophiel
Wow. Regular ol' Joph fan club in here.