Sitemaps, in simple English, are text files containing a list of all web pages that exist on your site. It is important that you create a comprehensive sitemap as it will help Google and other search engines crawl your site better and that might mean more traffic SEO coming your way in the long run.
That's a bad thing because some of your older blog pages, that are missing from the system-generated sitemap file, may never get indexed in search engines. There's however a simple solution to fix this problem.
Generate a Complete Sitemap for your Blogger BlogStep 1: If you haven't done this already, sign-in to your Google Webmasters account and add your blog. For details, check this video tutorial on how to add a blog to Google.
Step 2: Once your blog is added to Google, open the Sitemap Generator and type the full address of your blogspot blog (or your self-hosted Blogger blog).
As shown in the video screencast below, you need to manually add each of the sitemap files into your Google Webmaster Tools dashboard but for Bing or Yahoo, you can simply click blue hyperlinks and these search engines will automatically download the relevant sitemap files from your blogspot site.
switched from Blogger to WordPress, it still makes sense to submit sitemaps of your old Blogspot blog as that will only help search engines discover your new WordPress pages.
Hope this will help you all . Good luck...
credit : labnol