Google seems to have updated the user interface of the advanced image search page to look just like Google.com web search page.
The newly redesigned page is now musch easier to use, and it groups the search options and includes some of the options that were only available in the sidebar.
Why use the advanced search page instead of using the sidebar? Some of the options aren't available in the sidebar: restricting the results to a filetype, finding Creative Commons images, finding images from a domain or from a country.
Here's the old UI: