What is "watchful waiting" and why would I choose it as a treatment?

Treatment equivocal. In medicine there are many conditions that are either unclear at first or the treatment may be higher risk then observation. Then you just simply wait for disease to "unfold" and either declare itself or develop fully. Also, there is a number of people, who undergo "spontaneous remission" - the disease just regresses without treatment. So, 3 reasons: unclear, not worth treatment, may cure itself.
Wait and See. "watchful waiting" or observation is commonly utilized in medicine for many reasons. Often it is used in situations where the diagnosis is unclear or the risk of treatment is excessive. We can then "wait and see" if changes occur that make the diagnosis more clear and allow for a more specific and thus usually better treatment or when it is clear that despite the risks, the treatment is needed.