They are trying to get to the point where they scan everyone, but for now it's a random security check.
Usually they want to see everything, and it will depend on the security CM. Last time, I had one who wanted to look in my wallet. That, IMO, was overkill. But then same CM completely missed the front section of my backpack
My advice is to carry what you'll need and to not worry about security checkpoints. Don't neglect to carry important items, such as sunscreen, umbrella/ponchos, and water because of the security checkpoint. Just have your bag open and ready when you get there. Also, put everything in small, clear, plastic baggies so that they can easily see. Talking them through what you have also helps. For instance, one day on my last trip, I had a portable nebulizer one of the days when I was in the park, which most security guards probably won't be able to identify. I had it open and ready to show the guard right away and told them what it was. They took a brief look at the rest of the contents, but since I was straightforward with them, they pretty much figured I had nothing to hide.