Image resizer is intended to dynamically create new copies of the images on your page to the sizes you specify.
lets take the example of a products page.
in your site admin, you would have a form to enter product information, name, description, price etc. you also want to allow the admin to upload a product image, and you store the image name in a column of your table when inserted.
on the product results page, you may want to show a thumbnail of the image, and on the details page, show a larger version of the image.
instead of uploading 2 copies of the image, you can use image resizer to dynamically create the thumbnail on the results page.