Two-syllable phrases like "free book," "fast help" and "lose weight" are the kind of advertising messages that don't need to be read to be effective. By that I mean they are so easy for the brain to interpret as a whole thought that they're "read" in an eye blink rather than as linear verbiage. So for an advertiser trying to get attention in a world awash in advertising images, it makes sense to try this message-in-an-eye-blink route to the public consciousness--be it for a sales slogan or even a product name.
This kind of approach is particularly essential for billboards because drivers have little more than an eye blink's worth of time to glance at an advertisement and understand it before whizzing past. Newspaper and magazine readers do their own form of whizzing as they skim magazine or newspaper pages for whatever catches their eye.