Can anybody tell me that why we use meta tag in html
Meta tags describe your page. Search engines have famously used them to help index your pages. Its got nothing to do with asp.net but rather the HTML output that is produced. See here for more information. Be aware although that search engines rely less on them as they do now and incorporate other factors into their indexing.
Meta tags helps your website getting found by search engines like Google, Bing etc. When you use it the right way in combination with some landing page you can get found in the best way. Another method is the use of link-exchange. If you use the 3 methods it could get your websites way better indexed by google then if you only use the meta-tags.
If you want to know more about it, just search on google with keywords: landing page, indexed by google, meta-tags, link-exchange etc.