Philippa: Unless you're talking of a different "I'm here because I'm here" then surely it's unusual in being later than most of our nonsense songs (which might be expected to leave its words less confused). I know it as "We're here because we're here" and as a first (?) world war song; hence it's the humour of bored soldiers rather than somthing obscure?

But I could be wrong. I often am!

George