Google introduced Supplemental index in 2003 to show more accurate result for search query. Actually supplement pages are those pages that Google think important for most esoteric queries. Google also make it clear before they didn’t show supplement results simply by ignoring them. Google added supplement result in search results only to help web surfers to find more specific results and for that Google improved crawler frequency.
For more detail visit SEO Blog
Regards,
Bhavesh Goswami