One case where it’s important is when you’re blending together a thick mixture and a thin one. Imagine. for instance, that you’re adding a dollop of sour cream to a bowl of tomato soup. If you drop the sour cream in and stir, the sour cream will break up into little clumps, but will have have a hard time evenly dispersing in the soup. Do do it properly, you’d want to start with the sour cream in the bowl and gradually stir in the soup (this would be an absurdly anal thing to do for a bowl of soup, of course, but you get my point). The “thin into thick” rule is pretty universally applicable.