A Sitemaps main purpose is to help robots find pages of a site that might not be able to be found otherwise. Having a Sitemap would make sense if you have a site with high volumes of pages, bad structure or poor internal linking. If you’re Dave the local plumber and your site is only 5 pages, it is unlikely that you will need a Sitemap to help robots index your site. This is because a Sitemap is just a guide for the robots, for example; just because you have included all of your pages in your Sitemap does not mean that Google will index all of them.
John Mueller from Google has confirmed what seemed obvious when faced with a question on Google Web
Master Central regarding a small site and it’s Sitemap.
Google’s John Mueller:
“With a site of that size, you don’t really need a Sitemap file, we’ll generally be able to crawl and
index everything regardless.
Also, with such a small Sitemap file, you can just check the individual URLs to see if they’re indexed
Google will easily index a small website without a Sitemap, so it seems like there is very little point in building a Sitemap for a small site. What constitutes a small website though? 50, 100 or 1000 pages? The site discussed in the forum is only 21 pages, so we know that anything around that size or smaller should not have problems being indexed.
John’s comment is also assuming that the site has a very traditional navigation structure, should the site have separate areas and independent navigation a Sitemap would still be necessary to ensure as many pages as possible are indexed.
Certainly having a Sitemap is not detrimental, so it is best practice to include one. It generally only takes a few minutes and will ensure best practice for the future, should the site increase in size.
Read the full discussion on Google Webmaster Central