It can provide security through a commitment...although it isn't required for all couples, and can often be a false sense of security.
Actually, I don't know of any couples anywhere, who have a sense of security without marriage. No, marriage isn't a 100% guarantee of security in marriage, but smoking isn't a 100% guarantee of cancer either. Certainly having either ups the chances.
Just because you don't know any, doesn't mean they don't exist...not everyone requires a piece of paper to feel more trusting and secure towards their partner. It's not the predominant choice, but it certainly occurs in some relationships.
No, I know plenty that 'say' they are secure in their relationship. But I can tell they are not. They don't act like they are secure. They don't treat each other like they are in a permanent relationship. But oh yeah, they'll say they are. I've met plenty that 'say' they are.
In fact, I just had this happen a year ago. I met a couple from Canada, who were living together for several years. I knew them for years. Both of them said they'd never get married. No need. Why bother? It's just a bit of paper.
I even asked the girl (she brought up the topic not me), because we were friends for several years, I ask since they are not married, statically, he'll move on and find someone else to shack up with, wasn't she worried about that? She said no way, not a chance. They were dedicated to each other. I said ok.
Last year, he was gone. She went to work one morning, came home and the house was cleaned out. She can't even get her stuff back, because legally they both owned it all 50/50. So she can't go to divorce court and separate stuff, because they were not married. She just lost everything, and him.
Found out.... she freaked out on him, because he was talking to another girl. He wasn't do anything, she just couldn't stand him talking to other girls, and he had enough, and ditched her.
Why did she do that? She wasn't secure. They were not married, and she knew that deep inside, and knew he could leave at any time.... and she was right. He dropped her like a bad habit. Why not? He wasn't married, and made no promises.
So yeah, plenty say they don't need marriage. But saying you are secure, and actually being secure, are two different things.