Double Clicking Rule
For 90% of our designs, a double click is really unnecessary, but on some templates, a double click will allow you to access the object behind another.
Here is an example of when a double click is necessary.
To achieve a certain effect, we will sometimes place a shape and an image/pattern directly on top of one another (layering). We then lower the opacity of the image to allow the shapes color to bleed through (see below).
To change the color and access the shape tools behind the image you'll need to double click on the image. You'll know it has been selected if the left tool panel has changed from image tools to shape tools.
All that double click did was allow you to bypass the image on top and access the object behind.
You can achieve this same effect by understanding layering.