Because you never know what trivial bit of information may ultimately prove to be vitally important.
Hell, if I'm not picked, they used to tell us come back the next day, now the system picks everyone they need that first day, and if you're not picked, then you're free to go, thanks for coming to do your civic duty! I was kinda disappointed the last time I went, and the new system was in place, I barely had time to get thru one chapter in the book I brought along, then had to report back to my boss that I was their's again without interference.
This jury was going to be used for only one trial. So once they picked out their people, everyone else was cut free. I didn't get the word until 4:30 in the afternoon though. It was a long, boring day, and I had to pay attention the whole time because they kept asking us questions to weed out the undesirables.