Once upon a time people lived in something called the 'real world'. Instead of constantly having their attention focused on little rectangular screens, people went to real places with their physical bodies and walked around. Back then, if you were in an office or shop you could find other humans and it was possible to talk to them without the permission of the owner of the establishment. If they had something to say about the place you were in, they could do so. Even if they were a complete stranger, the very least you knew had in common was that you in were in the same location.
Hijack is a modest attempt to give back something of what we have lost from moving into the virtual world. It makes it possible to talk to the people who visit the same virtual places you visit without needing the permission of the creator of that web page.
If you have an experience with an organisation that you want to talk about it, you can of course tell your friends on facebook, but the chances are, they're not going to be interested. Often, the people that you most want to talk to are not people that you know, but people who also have experience with that organisation. Sometimes website owners do make it possible to talk to other visitors, but they only do this when it is beneficial to them, not necessarily when it is beneficial to you. Even then, they usually monitor the conversation and censor anything potentially embarrassing.
With hijack you can say what you like, where you like. So, if you went to a restaurant and had a great meal, you can tell people about that on the restaurant's website. If you have a less than brilliant experience, you can talk about that too.