The aspect ratio of an image can be very important as it decides the shape of the image and therefore how the image is displayed and cropped.
A correct image aspect ratio ensures the key parts of an image are shown to the user and not cropped or stretched.
In broader design, the aspect ratio determines how much space an element takes up on a page. With this in mind, aspect ratio is one important part of providing emphasis to specific content on a page and allowing users to focus on the key call to action.