Sitemaps help search engines properly index your site. The problem is how to create a sitemap for google and other search engines. The easiest way to create a site map in a wordpress site is to install the Google XML Sitemaps plugin. Installation and setup is quick and easy and once installed the sitemap plug in just sits in the background and does its job.
If your website is small and links within your site are well defined a site map may not be necessary. As your site grows, a sitemap becomes more important.