image resizer creates a new copy of the image based on your dimensions automatically and that is the file used by the page.
when you right click and open the image, you are opening the newly created resized image.
using the original large image and changing the size still loads the large image to the client, the client has to download the large image.
using a thumbnail is preferred.
image resizer automatically generates that thumbnail for you.
if you use the same image on multiple pages all with differant sizes, than you would need to manually generate the thumbnails. say you decide to change the size on one page, you need to generate another copy of the image.
using image resizer does this for you automatically rather than you having to create the thumbnails.