There was a time in the past when some services online wouldn't immediately authorize the card - it would only check if the value was a "valid" number.
So, you could, hypothetically, get 1-3 hours of shell internet access based on a generated number.
Wouldn't a "bad" fake (i.e. anything with a valid checksum) work just as well?
More importantly, the line between generating a good fake and fraud becomes thinner the better your fake becomes.