You're thinking of situations where you are able to see a whole object at once. If you were dealing with an object too large to see all of, you'd have to start making decisions about how to explore it.
The mental image I like: imagine you are lost in a hilly region with incredibly dense fog such that you can only see one foot directly in front of you. How do you find the base of the valley?
Gradient descent: take a step in the steepest downward direction. Look around and repeat. When you reach a level area, how do you know you are at the lowest point?
