For a donation of 30 euros, Palestinians in the West Bank spray paint your message on the wall. They will send you 3 digital pictures you can keep forever (long after the wall is gone). The money primarily goes to support local projects and organizations.
The Wall won't fall just because your text is written on it. True.
But your message reminds Palestinians trapped inside the Wall they have not been forgotten. You help to keep hope alive. 'Our' Palestinians want to send you one single, simple message: "we are human beings, just like you, with sense of humour and lust for life." That's why they do this, and enjoy it.