How does using BCC help reduce spam?

Is it true that if I use BCC to email attachments, it will mean less danger of spam? How's that?
It's not so much about attachments, as it is any email you receive and then forward.
And it's not so much about saving yourself from getting more spam, but saving the people that sent you the email you're about to forward.
It's all about keeping their email addresses private and un-harvestable.