Magazine inserts are an effective way(to deface magazines ) for companies(people) to distribute information ( pages from my note book) to a wide array of customers(people who don't know me). They can be used to introduce a new product(or publish semi naked women), publicize a special promotion or even kick off an advertising campaign.
I pinched the above from some marketing website, but nothing stops a resourceful artist from doing this themselves, as everybody likes free stuff .
They are all out there. http://tinyurl.com/3ezfsr5
