Two main concerns for any document imaging exercise are the image quality and the file size. The objective: get the best possible image quality while keeping the file size to a minimum. Thus image enhancement has become an essential step in a well defined capture workflow.
The purpose of image enhancement (image cleanup/image processing) is to make the images more readable, and also to remove unwanted noise reducing the storage requirements. This is especially important for forms processing/OCR applications in order to improve character recognition.
One of my 8 things authors -- Amila Hendahewa, AKA the DocuDude -- has done a nice piece on the 8 Image enhancement techniques in document capture -- it's a useful primer -- check it out. You can also follow him on Twitter HERE. Here's his earlier post -- 8 Things to Consider in Selecting the Right Scanner.