What is this need of announcing to other people that you're not single?

I’m talking mainly about social media (Facebook, Instagram, etc). There have been many cases where people who aren’t single anymore putting their picture of ‘themselves with their SO’ in their profile page. It’s different case if they put it inside an album folder or such in their account, but no, they put it on their profile page (the first thing people see when encountering them on social media).

What’s with this need of showing the public that you’re currently in relationship? I understand that each person is individual and has different reason but seeing that many people are doing this I believe there must be an underlying main reason why they’re doing this. Apart from showing off, I can only think that one of the person is insecure and want their partner to show their picture together in social media so that other people will not get close to their SO which they have already claimed. What do you think?