I think they are easy to use, and I have even seen people include a contact form in one. Are there any studies on the usability of these footers, such as a Jakob Nielsen study or any other study? Also what are they called?

Do any of you have these on your site currently, and how might they have helped your site?

I also wanted to know if these are mainly for SEO, such as to help increase indexing on large sites. Or are there other more important reasons?