It means that you've become a liability to the organization that you serve so it is better to leave out of your own accord than to be thrown out. This is the quickest way to kill the storm. Imagine what it would have been like if this had been drawn out over many months, it's bad enough as it is.
Resigning looks so much better than being thrown out so if someone has messed up but on the whole has done an ok job (or in the case of Petraeus even a pretty good one) then giving them the option to bow out rather than to be tossed under the bus is a good thing. For everybody involved.
Resigning is honorable, but he might have kept his job if he made a public apology. The American people are generally quick to forgive public figures whose sins have come out and then confess. Lance Armstrong, on the other hand, is not likely to recover his image.
1. http://www.usatoday.com/story/news/nation/2012/11/09/david-p...