That's the initial pool. It gets winnowed down pretty fast for any and every little reason by both the prosecutor and the defense.
The main reason they begin with such an incredibly large jury pool is to ensure they never run out of jurors through what appears might be a very long trial. If I'm not mistaken, if that happens, it's a mistrial and they must begin again.
Two years ago I served jury duty with a pool that began with 120. The case involved two people who were accused of robbing homes in the Mountain Shadows area during the Waldo Canyon fire. I was one of the many who were dismissed by either the prosecutors or the defense, ostensibly because I lived near there and had to evacuate myself.
At least that's what I'd like to think. Might have been that question about whether or not the electric chair was still an option...*