Newly hatched chicks have a tendency to fall asleep wherever and whenever. (even standing) Not to mention, they are very unsteady on their feet. So the marbles in the waterers help to keep the chicks from drowning. They can drown in very little water. It's just a precaution.
I raised chicks from a feed store and they did fine with no marbles. (I didn't know any better at the time) But when I incubate chicks, there is a huge difference in their steadiness on their feet for a few days. I put the marbles in just in case. It's not worth it to me to lose one.