If you have ever worked on a website, you must have heard something about Search Engine Optimization (SEO). More importantly, you will know that any website you create and publish is indexed by several search engines. All you have to do is make sure that you write semantic and structured HTML markup and any web spider will understand what your website is about and what parts of it are more important.
Search engines, such as Google, have indexed billions of web pages based on their contents and semantic markup.
Apps tend to be a little less neatly structured, and crawling them is a lot harder, if not impossible. There is no formal way to figure out what content is on screen and what this content means. Also, more importantly, a lot of content you'd want to index is only available to users who have logged in or created content...
