Answer:
To inform you (the reader) of what to do and what not to do.
Explanation:
By starting it off by saying "..you must consider how you design it" and then following it up with an image of a good and bad form for an App, it gives a visual idea of the Do's and Don'ts, which can help you out much more than explaining it in text.
Hope this helped!
Source(s): None, except my mind. Hope this helped!
