I was a juror on a murder trial just a month ago (full/long story here: http://rants.granroth.org/2011/04/the-trial/). I have wanted to be on a jury for years, though.
Why? Well, any society will eventually veer towards tyranny. This is the natural order of things. There are only two avenues of power that an average citizen has to prevent this from happening: voting and juries.
The reasons for voting are pretty obvious. They are the way for otherwise powerless citizens to influence what those in power do. As time goes by, though, the power of the vote diminishes... at least here in the US. We're at the stage where nearly all candidates to any political office are controlled almost entirely by corporations and other special interest groups. It's very much a "damned if you do; damned if you don't" situation.
That leaves jury duty as the sole incorruptible source of power for the average Joe. The entrenched power structure will always try to use the laws to keep the powerless under control. The ONLY thing keeping that from happening entirely is the jury of your peers standing between you and those that would suppress you. A jury, then, is there to protect the defendant as much as possible.
Now... I'm not naive. I realize that many (most?) juries exist just to put a rubber stamp on whatever the prosecutor is telling them. But it doesn't have to be that way. If enough principal minded citizens go out of their way to *not* be excluded from the jury, then this can change.
I do realize the perceived hypocrisy of me talking about protecting the defendant when, in the one case I was a juror on, we judged him guilty. I don't see that as a problem, though. A jury doesn't exist to subvert justice but rather to ensure that justice is served. This guy was definitely guilty.