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.
Below are a few examples of when a double click is useful.
To achieve a certain effect, we will sometimes place a shape and an image directly on top of one another. 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 shapes 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.
It's important to first understand that all of our frames regardless of the objects inside them (i.e. Circle) are always in the shape of a rectangle.
This rectangle actually represents the clickable region of that object. On a circle you could actually be clicking on something else outside the circle and still selecting the circle. This is a problem that most editors have and one that is easily resolved by simply understanding what is going on.
The example screenshot below will provide a visual of what is mentioned above.
The dotted line represents the actual clickable region of the circle image (note that this dotted line was placed there only to exaggerate the clickable region and is not actually visible in the editor)
The pulse hand represents a potential click you may make. If you were to click there with the intention of selecting pink circle. You'd actually end up selecting the lady doing yoga. Now in this example, you shouldn't have any problem simply moving the mouse over to select the pink circle, however, in other layouts with other shapes you may need to use a double click to select that region.