This isn't about traffic analysis, it's about social expectations and social norms. If privacy is the default, the social norm is to be private, and to expect privacy. That's important.
I guess that, in this particular case, the reason for the envelopes is to conceal the ads inside them until the recipient has taken the time to open the envelope.
Kinda like this:
http://thumbs.dreamstime.com/z/sale-advertising-papers-15592...
The advertisers pay bulk rates to USPS to stuff all this crap directly in our mailboxes.
There are some exceptions which arrive in envelopes, mostly to trick you into thinking it isn't just spam mail like the rest of the crap.
Not targeted for your reply, just clarifying the previous post: I don't appreciate the down votes though, I wasn't stating that OP opinion is right, it was just my understanding of what he meant and trying to understand it.