It is best for a website or blog to have 2 sitemaps one is for people visitng your site which is a regular html page or blog page.
The other kind is an XML Sitemap which helps the search engines Google, Yahoo and MSN index all the pages of your site which may be dificult for them to find.
– Search engines can not crawl easily pages that are Ajax or Flash rich.
– You may have a new site with few links pointing to it.
– Some of your pages are not linked to each other pages or not linked
– Your site contains dynamic content.
An XML Sitemap allows you to give the search engines Google, Yahoo and
MSN additional information about your site:
– How often your content is updated
– The date on which a page was last modified
– The importance of pages on your site relative to other pages on a
scale of 1 – 10.
Google also indexes sitemaps that are specific to Google and not recognised by the other search engines. These special sitemaps are for websites that contain mostly Videos, News, Code Searches and Mobile content.
Generate Search Engine Sitemaps
There is a good plugin for WordPress blogs that will automatically generate a normal xml sitemap and a gzipped one for your blog and notify Google, Yahoo and Ask that you have updated your blog.
You may even manually add pages that are not part of your blog but are on the same domain.
Download this XML Sitemap Generator Pluign
Using sitemaps on your website is good SEO practice.