The file size of digital photos these days is usually quite large. This means that there is a great deal of information in the file. To present a picture on the web or to email a copy to friends and family it is best practice to resize the photo to something more manageable.
Nearly any photo editor allows you to resize photos. Outlook Express will even offer to make your files smaller for email. Most photo editors also allow you to crop the photo.
Cropping the photo allows you to select only a portion of the original file to save as your picture. This allows you to cut out some background that may detract from the picture. If the photo was not framed as well as it could have been sometimes you can correct that by cropping the photo.
As long as there is enough detail in the photo, cropping the photo can also have the effect of enlarging the subject of the picture. If, for instance, you take a picture of a flower and the flower takes up a quarter of the frame you can crop the other three quarters of the frame out and end up with a full frame picture of your flower.
By cropping your photos you can focus on the subject and make your pictures more interesting. You get the added benefit of having a smaller file size. The pros do it all the time, why shouldn't a serious amateur photographer do the same?