It's not exactly the same thing. To serve targeted ads, you need to associate the information you inferred from e-mails with individual users (and with so much

It's not exactly the same thing. To serve targeted ads, you need to associate the information you inferred from e-mails with individual users (and with so much data that goes through your Gmail account, that means building a very accurate profile of the user: their interests, buying habits etc.). To filter out spam, you don't need all that. In fact, you can build a pretty decent spam filter that is entirely contextless i.e. classifies e-mails without knowing who's the recipient.