The reason why the server behavior only refers to the cache folder is that you can actually apply it to the entire page at once.
If you have a page and you want to use "crop from center" or "fit to box" on all images on the page, then instead of applying it to each image you can actually apply it to the entire page body and all images will automatically use the resizer... just change the width and height in the image property inspector and the rest is automatic.
This is also why we didn't include the width and height in the resizer itself.