Can somebody explain to me why I should possibly use ImageMagick (or the fork GraphicsMagick) in a CMS instead of simply displaying and sizing my images via CSS?
The browser methods cannot fail and are automatically updated on client side, the ImageMagick-binary can fail on the server and I have to maintain it by hand to not become obsolete.
ImageMagick offers lots of image manipulations not available via normal HTML/CSS operations. So for some tasks, your browser just can't do the modifications.
One very important task is simply file size: if a user uploads a 20MB image you don't want to deliver that to your clients with a 3G mobile connection and let them scale the image down: you want to have your server do that task once and then serve images that are substantially smaller in size.