Why is convertd still active, when I have attachment indexing turned off, and users are only allowed to view attachments in their original format?

I seem to remember there's a good answer, and I think I even saw it once upon a moon, but have utterly failed to find it again. I have searched the documentation, the forums, the bugs. Please help me to understand:

a) what convertd does when indexing is turned off
b) how convertd fits into the mail flow