Typically they use the email to spam the crap out of your email and usually under several aliases. Some times it's for some other scam they want your $ for or like Russian Roulette or when you click on it or a link in it, your computer gets compromised in some fashion with spyware etc.
I am constantly getting after my wife about not opening email from anyone she doesn't know. Last year, she called me into the office after she clicked on an email and a pop up came on the screen she couldn't get rid of. About a week or so later, both hers and my credit card sites got hacked and all our reward points were gone. After we noticed what happened, I did a serious scan of the computer and found a software program on the system that tracked keystrokes. Deleted it and no problems since.
The other scenario is the scammers want to go to your private email to continue the transaction off of CL. You have seen the ads like: 2009 Harley Electra Glide ($3,223) or some other strange amount. There's only one small picture and the description isn't even close to the way a typical buyer would describe their bike.
When you get more info, you are told that they are in the service and out of the country yada yada yada