You can extend it even more, for example. If you have low bandwidth web hosting, you don't want google to index your images. So it's a good idea to disallow access to your images folder, your css, themes, etc.
That is a good suggestion indeed. I did not thing of that before. Unless of course, your images are important to be indexed.
yes its perfect use disallow and if u add / then google will not crawl that page and u can use it like this if you want to crawl all pages.. User-agent: * Disallow:
Basically you want to disallow on all pages but the ones that product good content. For example on my community I've disallowed pages like profiles etc. all pages with useless information on.