Often there are cases where you need to add images with a custom size, as your theme doesn't offer the predefined size. This plugin enables you to do so with a shortcode or a template tag. Or maybe you just want to avoid that WordPress core and your theme generates all the pre- and theme defined sizes and just use custom ones. Another use case might be to directly include it in a theme - the MIT license allows you to do so. And last, but not least, it also serves as replacement for TimThumb.
WCM/DIR can be used as standalone Plugin, or MU-Plugin, but as well serve as included tool in a theme or any other plugin.