Importance of Sitemap for Blogger blog

One of the common question in Blogger Help Forum: How Do I?, we see evidence of confusion and doubt.
Do I really need a sitemap, for my blog?
This question, when asked, may help us to design our blogs better.

 WikiPedia defines a sitemap as
a list of pages of a web site accessible to crawlers or users
Classically, a sitemap is a visual index, to help the people viewing a static website, to easily identify and access a specific article in the website.

 A sitemap provides an alternate index, to a non Blogger website. Most websites are static, with a hierarchically accessed, single structure. A well designed sitemap allows people and crawlers to more efficiently locate specific articles (pages or posts), in a static website.

 Most Blogger blogs provide much more than a hierarchical, single, static structure. By default, a Blogger blog links its pages and posts dynamically, using several alternate indexes.
Archives (posts listed hierarchically, by date).
Labels (posts listed hierarchically, by topic).
Extended main page (posts listed sequentially, by "Newer Posts" / "Older Posts").
Any blog, which provides one or more of these features, is effectively its own sitemap - for viewer access.

 For crawler access, the options are simpler.
Archives (posts listed hierarchically, by date).
Extended main page (posts listed sequentially, by "Newer Posts" / "Older Posts").
Label searches, which use a different URL structure from the base blog design, are not used for indexing a Blogger blog.

 If you look at the main page of a typical blog, you can follow the "Newer Posts" / "Older Posts" links from page to page, repeatedly - and eventually, see every post in the blog. With a new blog, with few posts, and predominantly "first visit" viewer traffic, the main page makes a passible sitemap. For an older blog, with many posts, and more "repeat" viewer traffic, the main page makes a less efficient sitemap.

 For crawlers, which will generally follow a limited number of links within any single blog or website, the main page makes a still less efficient sitemap. Most blogs with any appreciable search engine reputation, however, only need new posts indexed by the crawlers - as older posts are already indexed, and remain in search engine cache.

 Blogger provides a good, default gadget which serves as a sitemap, on most blogs - the Archives index. Look in the sidebar of this blog, about halfway down, for the "Contents" gadget.

 The Archives index is an HTML based gadget, which produces a set of hierarchical, date structured links, exhaustively enumerating each post in the blog. It's an ideal structure, for search engine bots (crawlers) to follow. If your blog includes this accessory, that's probably sufficient for indexing.

 Blogs which contain one or more complementary features can provide search engine access, organically.
Well written, regularly published posts.
The standard Archives gadget.
The standard "Newer Posts" / "Older Posts" links.
Such blogs may not need a sitemap, to provide good access to both people and crawlers.

 Both the Archives, Labels, and "Newer Posts" / "Older Posts" links are subject, on some blogs, to customisation. A blog with Javascript driven Archives, Labels, and / or custom pagination ("Newer Posts" / "Older Posts") gadgets may not provide easy crawler access. This also affects blogs which use dynamic templates. If you tweak your blog extensively, you may want to consider these issues.

 There is one special case where a sitemap is always needed. Any time the URL of your blog is changed - whether to a new BlogSpot URL, or to a non BlogSpot custom domain - prompt re indexing, under the new URL, is a necessity to regain search engine reputation. A robust sitemap set, which directly references all posts in the blog, helps the crawlers to re index each post, under the new URL, much faster.

 Other than that special case, a blog with standard, well designed, features may not actually need a sitemap. If the blog does need a sitemap, you use Google Webmaster Tools.

 A sitemap, setup through Google Webmaster Tools - and using the blog posts newsfeed, defined through "robots.txt" - is a simple accessory to add, and requires no ongoing maintenance. Given this reasoning, most blog owners simply setup a sitemap, and don't worry about the above issues and questions.