If you generate a xml Sitemap for submission in webmaster tool, you do not need to update it every time you add a new page to your website. The sitemap gets updated automatically.
Using sitemaps has many benefits, not only easier navigation and better visibility by search engines. Sitemaps offer the opportunity to inform search engines immediately about any changes on your site.
Yes, you have to update your sitemap on regular basis. Sitemap gives information to web crawler about your pages. If you are updating or adding new page then it should be informed to web crawler.