The point is to get the most texel to pixel ratio. Pack Island is a quicker way or sometime a starting point to get the best ratio. Manual placement takes longer and sometimes is the better option. For example: doing a low poly soccer ball. Even with simple colors like black and white, if the islands are rotated at certain angles you will get jagged lines when painting or baking the colors. In this case, I did pack island to get a starting point and then manually go through and rotate the problem areas to get straight 90 or 45 degree angles on the outer edges.
