I think it really depends on the industry and scale of a project. I'm not a marketer, but I imagine a social marketing campaign is fairly easy to start and/or stop without serious repercussions for either party. I have also never used contracts with any freelancing web dev clients I have had, but they generally have been fairly small in scale.

In contrast, years ago when we lived in the states I worked as a project engineer on several highrise construction projects. I did A LOT of managing contracts and making sure that obligations were met. In this industry contracts are essential because failure to complete requirements on time can cause a domino effect that can lose millions.