The idea behind the apron is that a digging animal goes up to the fence, starts digging, hits the apron, and does not know to back up. They are really effective and a lot less expensive and time consuming than covering the bottom of a larger run with mesh. Easier on the chicken’s feet too when they scratch.
I have had snakes eat eggs and baby chicks. How big a chicken a snake will eat depends on how big the snake is. Most adult chickens are really safe but chicks of varying size and some small bantams can be at risk. It just depends on the snake.
Rats will tunnel. Raccoons might dig though they are more likely to try to get in by climbing. Foxes, dogs, coyotes, skunks, maybe bobcats are more likely to try digging. I don’t know where you are but even in suburbia you’d be surprised at how many of these critters are around.
Another risk with a fence, depending on how the bottom is finished, is that if it is just wire many predators can just squeeze under the fence with very little, if any, digging. All it takes is a small opening. Many predators are mostly fur. You’d be surprised and how small a hole they can squeeze through. An apron stops that.
My chickens forage inside an open electric netting and I have hawks all over the place, some pretty big. I’ve never lost a chicken to a hawk, though an owl got one when I was late locking them up. Some people have a huge problem with hawks. Predators are like that, they are a constant risk but they don’t always attack. That’s why when someone like me says they’ve never had a hawk attack it doesn’t mean you won’t.
What kind of mesh do you have on top of the runs? Metal wire, plastic, cloth? In theory a hawk could dive bomb through some weaker mesh and get to the chickens, probably could not get back out through that hole though. But that is unlikely to happen. A hawk’s bones are pretty weak and if they ever break a major bone they are dead. A hawk is pretty fierce in the attack but they are also kind of cautious. They don’t want to get hurt themselves. As long as they can see that mesh they are unlikely to hit it.