The flyer campaigns offer WTS a piece of time where only cheap flyers are left with people. That means less distribution of thicker materials. In the past, they wanted the JW's to pay for the thicker materials and try to get some donations for them from the public. As time went by, they realized that JW's were giving their time and only putting the public-collected donations in the box without donating much themselves. The members figured that Jehovah was blessing the work, the important thing was to get "the truth" into people's hands and then they would contribute.
So a month or so of only distributing cheap flyers (that the dubs generally do donate a small amount for) allows WTS to recoup some losses. Members do donate at the conventions, so they feel better about trying to get others to the convention and may even donate a tad more.
In the case mentioned about rural areas 7 hours away from the convention, it is totally ineffective. In most other areas, a few curious may come but it really is about inviting the choir to join in the singing.