Actually Yahoo Explorer is not used for single purpose of knowing which or how many pages indexed and backlinks for site. Even both these we can find easily by entering query in the Yahoo.
For Links we can use linkdomain:www.domain.com
For Indexed Pages: site:www.domain.com
As for as Yahoo Explorer is concern here is the details why it is used.
Yahoo! Site Explorer is a tool that lets you access the information we have about a site's online presence. You can see which sites and subpages are indexed by Yahoo! Search, track sites that link into webpage, and view the most popular pages from any site. Use Site Explorer to:
* Find which sites and subpages are indexed by Yahoo! Search.
* Find inlinks or pages that link to any site or any page.
* View the most popular pages from any site.
* Submit and track feeds for your sites.
You can get even more detailed information and greater ability to manage a site or its URLs when you authenticate the site. You can:
* Find what subdomains of the site known to Yahoo!.
* See other information that Yahoo! detects about your site, such as language.
* Delete URLs that you don't want indexed by Yahoo!.