Update May 2015: Still being a very popular answer, I recommend you look at @alden's answer as it is much more updated then my own (6 years old now)
Original answer follows:
To tell the browser what the content type and encoding is
See http://www.mattcutts.com/blog/google-supports-meta-noodp-tag/ for why
Obvious
Google do not use this, but other search engines may
If you know when you like this page to be expired from cache
