The reason why a sitemap is requisite is that it helps the visitors to understand the structure and layout of the website, so he can get access that what the website is offering.
The sitemap is actually a page where the visitors can clearly see the structure of website and with just one click leap to the page they want to.
Apart from the human visitors, the sitemap is also advantageous for the “robotic” visitors from search engine. So being a web developer, getting all of public pages of your website, should be your main objective.
Since the site map is the page that carries the links to all the pages from one page, it deserves to be submitted to the search engines as it helps in accessing the indexed web pages rapidly by the search engine.
Most of the search engines limit the depth of searching on a website from the homepage. Assume that you have a page obscured deeply inside your website structure, such search engines not captures it. If the sitemap is submitted to the search engine, they can go to the page directly instead of pursuing for links.
The large websites might have hundreds or even thousands of pages. In such sites, to make give the easy and quick access to the human visitors to the page they want, the developers might create navigation or drop down menu but this navigational structure is beyond the understanding of search engine so for such websites, it is very important to have sitemaps.
With a site map you can plan you site exceptionally even before its creation. Once you draw a layout of your site, your job will become much simpler and the designing of web pages will be very easy for you.
In the nutshell, the sitemap is crucial for every website and it should be the first step while planning for web site.