November 2017, rev. December 2017
Many problems don't have obvious solutions. Which means in general one can't be sure ahead of time if a plan of attack will solve a problem.
But that a plan may not yield a solution doesn't mean one can't make claims with certainty about the plan. It's possible to not know with certainty if something is true but to be able to claim with certainty what the consequences will be if it is.
For example, if the success of a plan depends on x being true and the value of x is unknown, it's possible to claim with certainty that if x turns out to be true then surely the consequence will be y.
This is a powerful property, because it allows for progress in the absence of complete information. What-if questions help pick among plans when you don't know if something is true.