Here is why...
The biggest “real” problem for Cain is his lack of experience, by bringing Gingrich on as his VP he basically removes that objection, because the thinking would be that Gingrich would be there to help him or in other words... Gingrich would bring... gravitas to the ticket :)
Much like Cheney did for Bush.
But only if you think Gingrich could be President. IF not, there are certainly other people around who also are well-read, well-spoken, and could be the mentor for Cain, and wouldn’t have the Gingrich baggage.