The same way that posting 'i feel like killing myself', in its simplest form, does... it's an expression of emotion, of feelings, and by expressing it, it means that others can see it, and people don't have to be alone in those feelings.
In my opinion, almost anything someone can write on here, is preferable to actually doing it (as long as it does not harm others in the process). While someone is still talking, they are not killing themselves, they are not harming themselves, and they are open to other things, even if they are in the depth of despair. By posting what people think they would include in a suicide note (note, very different to actually posting an actual suicide note), people can understand their own, and others, core 'truth', the most important thing that they express, as it's the last thing they want others to know of them... talking about that, and how to gain more from that truth in life, than in death, is pro-life in my opinion.