Re: Erm
"How does this relate to the corresponding passwords getting lifted?"
Assuming you're being serious, I had hoped this was obvious: if not, then I've failed as a sub-editor.
If, say, you have a Dropbox account and a CrappoMail web mail account, and you use the same email address and password for both, and then CrappoMail is compromised and the hackers have your email address and password - they can log into the Dropbox account.
From there, the hacker can find a document with Dropbox users' email addresses. These are then turned over to a spam bot for fun and profit.
C.