In my point of view, the exercises of sorting are very important. It seems very basic or a simple loose of time, but it helps to practice the vocabulary. Plus to it, it happens that the small children associate the image of a flash card with the determined object or colour. For exemple, if the jacket of a flashcard is green, they can colour green all the jackets in worksheets. The same happens with animals or they also can confuse or mix up the meaning , etc. So, I think that more objects of the same kind are identified, sorted by shapes, colours, other characteristics it is much better for vocabulary skills.
Here there is an exemple of sorting activity.