What is a Site Map and Why do i need it

A site map (or sitemap) is a list of pages of a web site accessible to crawlers or users. It can be either a document in any form used as a planning tool for Web design, or a Web page that lists the pages on a Web site, typically organized in hierarchical fashion. There are two popular versions of a site map. An XML Sitemap is a structured format that a user doesn't need to see, but it tells the search engine about the pages in your site, their relative importance to each other, and how often they are updated. HTML sitemaps are designed for the user to help them find content on the page, and don't need to include each and every subpage. This helps visitors and search engine bots find pages on the site.
The key message is in the second paragraph: “Web crawlers usually discover pages from links within the site and from other sites. Sitemaps supplement this data.

Here is what Google itself says about Sitemaps:

Sitemaps are particularly helpful if:

•  Your site has pages that aren’t easily discovered by Googlebot during the crawl process – for example, pages featuring rich AJAX or Flash.
• Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so if your site isn’t well linked, it may be hard for us to discover it.)
• Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.

Unless you fall into one of those cases, therefore, having a Sitemap will not be “particularly helpful” for your site.

In fact, some people claim that it might even be the opposite. There are cases where the addition of a Sitemap actually reduced the crawl rate of the website, possibly because Google no longer need to crawl the site completely to discover its internal pages.

If your website is having indexation problems, has a poor internal link structure, or falls in one of the three situations described by Google, then using a Sitemap would probably be a good idea.