I have read that in magazine. They want to prove if your email address is active (used daily).
You’ll get a reply from a human who will elaborate the scam further. I read an interesting interview with a Nigerian scammer, who revealed a really interesting fact: a lot of them speak much better English than the terrible grammar and clumsy pitch in the emails would indicate, but since they actually get so many responses anyway they deliberately phrase the pitch in such a way that only a fool would fall for them. In which case, the responses they get back are from those fools who are inherently easier to scam. So despite what I said earlier about phishing, you do actually have to be an idiot (or otherwise deluded) to fall for 419 scams, because they’re actually screening for exactly that.
Incidentally, if you want to see the hilarity that results from people who do reply to the scams in order to screw with the scammers, check out 419eater.com