I guess that a lot of people do it for closure, the same way that people write suicide notes. Video can express emotions that maybe they weren't able to write down.
If they wanted revenge, it would be a great opportunity to show people how broken they are, which might make people feel guilty.
Maybe, they care about some people and want them to see how genuinely sorry they are. Maybe writing some words on a piece of paper seems too impersonal and they want people to hear their voice, and remember it.
Maybe nobody cared about them and they wanted people to watch so that at least people would be aware of what was going to happen, and they really wanted that comfort.
Maybe, they love someone and they want that person to stop them and tell them how the world would be worse without them, but they're too scared to talk to them out of fear of rejection, and they hope for that person to see...
Maybe, they wanted to be famous for dying.
Maybe, they were angry at society and wanted people to see and be haunted by the effect that society can have on people.
Maybe it was an act of protest.
Maybe, someone told them that they were too weak to kill themselves, and they wanted to prove them wrong...