I'm not entirely sure what your asking, but I think this is the answer: Place this in your robots.txt file: Sitemap: http://www.example.com/sitemap.xml
I wanted to ask the usefulness of inclusion of sitemap in the website. It adds up another page and therefore expenditure. Does the crawler place equal importance to the sitemaps as to the other pages?
The sitemap is literally a map for the crawler. The crawler picks up your sitemap, and finds all other indexable pages on the domain. I don't understand what you mean by another page therefore expenditure. Depending on what CMS you use Sitemaps are super easy to generate. If you don't have a CMS, get a free Attracta plan, that'll generate a nice sitemap for you that is hosted on their servers instead.
Submit the XML sitemap in Google Webmaster Tools. You don't need to link to it on your website. Is that what you're asking?
You don't need to have a link to an xml sitemap anywhere. It makes no sense. Having it in the Robots file tells crawlers where to look for it since the robots file is the first thing they look at when crawling a domain.