Search Engine Submission is to inform Search Engines about the existence of a site. It is important that search engines do know about your site (index it) and submission is a way to make it happen.
Search engines usually have a link to accept new sites. It is very simple to submit a site in most of the search engines. There are some free tools also available to help you. Some tools submit a site to hundreds or thousands of engines at one go. So it is very handy. Webmasters can either submit a single page of a site or submit the whole site using sitemaps.
Every webmaster wants to get their site ranked in the first page, because that gives more possibility that people will visit their site. This is why they want search engines to index their site well.
When a site is submitted, search engines start to crawl it. When a new site is generated (or domain name has been changed) search engines can start to crawl a site only if the site is submitted or if there is an incoming link from another site. If none of the conditions are met, search engines remain blind about the site. So it is important to submit a site to search engines at the time when it was aired or if domain has been changed.
Since 2004, crawling technology has improved significantly. Search engines can identify and crawl new sites, even if it is not submitted. There are probably more than 100,000 search engines these days, only few hundreds of these are actually authentic. Others have been found to be in bad practices. If a site is submitted to those search engines, they generally use to send virus attacks or such malfunctions.
Ninety nine percent of search operations are done in top 200 – 300 search engines. Therefore, submitting sites to thousands of search engines, do not add any value, rather risks the site itself.
Therefore, search engine submission should be done only when site is brand new or when domain has been changed and must be limited to major search engines only. Once it is done, it is not required to resubmit, unless domain has been changed.
Sources: Google: Technology Overview, Wikipedia
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