My kids sometimes argue that they are never going to need to use their math.  I beg to differ.  Printing and framing your photographs is a good example of real like application and this article helps to explain why. 

Digital images from a cropped sensor camera (which is what the consumer dslr and point and shoot cameras are) produce prints with a 3:2 ratio.  (full frame or pro cameras have a 4:3 as a rule)  That means they print a standard 4x6in photo.  However, that also means the next size "up" is not a 5×7 nor even an 8×10.  It is an 8×12. (16×24 and so on)  That is a bummer because photo frames are not typically those dimensions. Most online photo labs will give you a crop tool to use as you select sizes.  It is still helpful to have a fair idea in your mind of what different ratios look like so you can plan enough margin into your shots to all for the inevitable cropping of parts of your image for framing.

 I had the advice "fill the frame" drilled into my head early on in film days.  It isn't such good policy for digital.  Zoom in close enough to isolate your subject, yes, but leave enough room around the edges to crop comfortably for enlargements while retaining all the essential elements of your image. 

(I am mid-relocation so if my math was off I beg pardon <g>  It is all in the linked article.)