Twilight Struggle

Instead of derailing the Tournament thread, I’ll ask my questions here:

A very interesting strategy guide has been mentionned there. I have done some reading and had a question tied to my experience with the game.
I noticed in my game with Tylertoo he was focusing as the US on getting a lot of European non-battleground countries under his control, starting with Greece and Turkey. I mimicked that behavior in my own game against Greatatlantic, not really knowing what I am doing, I must confess. Then I read in the aforementionned guide early war chapter:

you get two of the three critical Mediterranean non-battleground countries

(My very educated guess is that Spain is the third.)

Could anybody explain to me how and why those three countries are critical? The guide mentions European Domination; is that the only concern?