I only just added one recently. Site maps are great for websites with multiple pages and information. So maybe someone wants to find out about pets - site map will have the clik to that information (I just had a Dr call who is visiting our hosp and must bring his dog I told him about our boarding kennel/B&B for dogs nearby so he might go with it, but could not understand why WE would not accept them!)
Anyway, if you have a lot of 'hidden' pages, info that is a linkede page but not on your header or side bar you can site map them. I only have so much space and so many spots for titles on my header bar so the site map is one place to find/get to all the information.