This is an interesting question. By the way I have been lurking on TPR for some time and decided this was as good a time as any to sign up.
Regarding certain security measures, like metal detectors and back pack checks, a lot of it is simply deterrence. Someone who really really wants to do bad things will always find a way and you can never ever stop them. But most bad things are perpetrated on a whim, crimes of passion if you will, by people who don't go out planning mayhem.
For example, I work for an online Amazon retailer here in the Pacific Northwest. We have certain security measures in place that honestly don't do much at all to stop theft but do act as deterrents to theft. First off we are given a cloth mesh bag to carry our lunch in. Lunch must be in a zip lock bag or clear Tupperware type container. No backpacks, hot-cold lunch sacks, personal dishes or utensils, not even books to read on break. We have a fridge, microwaves, dishes, utensils, coffee, tea, and condiments in the break room. Any food brought in must be eaten that day or left in the fridge with your name on. It can never leave the building again. This is to make sure we aren't stealing food but does it really work? We can bring in all the pre-packaged food that we want but we sell pre-packaged food so someone that really wanted to could just steal it and eat it at work. But the average employee is deterred.
As for security when we first come into the warehouse we have to remove our keys and phones and turn our pockets inside out. Sunglasses and wallets and any other personal items must be left in our cars. Then we are wanded in. When entering this is be sure no one is bringing weapons and hurt others. But does it really work? We have things like box cutters and scissors in the warehouse. Someone really determined could do a lot of damage but the average employee is deterred. Then when leaving the wanding is to be sure no one is stealing. But does it really work? 99.9% of the items we sell small enough to smuggle on our person aren't metal, and those that are metal are super cheap. Also they don't check our back pockets. They let us hold our keys and phones in our hands so theoretically we could hide something in all that. Or down our underwear. In our shoes. Someone determined will find a way. But for the average employee it acts to deter. It's psychological. Just the idea that it might maybe detect something. I think it's the same at parks. It's a deterrent against he casual bad person and makes people feel safer.
