Thinking about this point more, I have an answer using another theoretical situation. Lets say a rogue branch of the military takes over Country X, and says their leader is Bob. Now if the president only meets with Bob, it could help to validate his claim as leader. And in this case, this is where a precondition could be important. The president decides to agree to talks, only with the precondition that both Bob, and the actual leader of Country X are both willing to sit down at the same table. This doesn't give Bob a position of strength above the existing leader of the country, as the president is only willing to meet with both to negotiate. And it also doesn't give the message that the president is automatically agreeing with the existing leader.

The Office of the President of The United States is a very important position based on it's power, even if it is currently occupied by someone that the majority of Americans do not agree with. If the president simply flies to some country, it can have widespread impact on many areas, from financial, to political stability of an area.