It will index your pages in yahoo search engine.It means if anybody search your blog in search engines , they will get it. It also helps to find any errors in your site ,popular page and what search engine queries. In the following post , you will get step by step instructions how to add sitemaps in yahoo. See also Submit sitemaps in Google.
Step 1: Visit Yahoo Site Explorer and sign in with your existing yahoo account.
Step 2: Add your blog (http://example.blogspot.com).Click on Add My Site.
Step 3: Now Authenticate your site by adding meta tag on your blog or site.
Step 4: Now add meta tag just after the (head tag) in the blog.Don't know how to edit HTML in blog, see this post Editing HTML In Blog. Click Ready to Authenticate that you see in Step 3.
Step 5: Now click on explore button just after blog name.Then you will see a page just like image below. Then click on submit it for free.
Step 6: There are two options (a)Submit a website or webpage or (b)Submit site feed. First choose option (a). Enter your blog Address.Then choose option(b).Enter your blog feed address. Normally the feed url is (http://yourblogname.blogspot.com/option). Here option may be one of them: