Downloading large bitmap images from the Internet over a slow network connection can be frustrating, especially over a slow mobile connection. It might be that the image you are trying to view is not the correct one but you would have to wait for the full image to be displayed line by line before you are sure

However, with interlacing the image is downloaded by alternate lines. This results in a blurring then smoothing effect. Roughly the same amount of data is transmitted, just the order is changed.