If I am a product photographer creating images of toys for sale online and I provided this image to the manufacturer, it may not be sufficient information considering this helicopter is a model kit. Yes, the website can provide measurements...but the sale may be more successful if the following images were provided.
The hand adds scale and a new sense of difficulty to the model being sold. However, is that a woman's hand? A child's? A man's?
This image in addition to the image with the hand would be satisfactory scale portrayals for this product. What about the next image? How large are the Storm Trooper, Obi Wan and Yoda?
What about if they started playing limbo with a cigarette? The reference in your mind of the size of the cigarette completely changes the scale of the image.
What about on a larger scale? You may have to use a human for reference. Here's some images from the web:
Scale is important in all mediums of art. When you attend a museum, information provided with paintings provide the title, artist's name, the medium and the size. This is relative when analyzing detail and technique. Many images of art shared online go uncredited without this information, which makes appreciating the work of the artist that much more difficult. The image below is an oil painting on panel by Eloy Morales.
Eloy is able to create this hyperrealistic effect by adding small details over a large scale. Cover the blow image with your hand, without knowing the size of the painting, you may think its a photograph, because the details are panned out to a larger image. The painting is actually 5'x5'. This is why full credits are necessary when displaying art in photographic images.
Let's take a look at another image which absolutely needs a sense of scale to appreciate the technique:We've visited examples of scale in photographs, paintings and drawings...but it is also useful in movie props. You've probably seen 1997's Men In Black, the scene with the dying Arquillian prince shares his last words is pictured here:
Your eyes will fool you! Scale is used both to create context and to add detail...in any medium it is applied!