and lots of other uses.. I use Zend's barcode library to create barcodes which are saved as images for printing labels etc, you can also (generate &) display emails as images to avoid scraping etc, creating thumbnails of user uploaded images, I have a dodgy label printer which I can't get working with the PHP printer draw functions but can get it to print an image so I create the image (serial/job numbers etc) and print that instead.
I am currently working on a valuation system that will show e.g. a picture of a diamond and then (how I have not worked out yet
) allow the valuer to mark where the inclusions are, then I will combine the 2 images to produce an image for the valuation report.
To be fair I mostly use imagemagick rather than PHP's image functions but either way its useful from time to time.