Search Engine Submission - is a process webpages owner to submit their pages to search engine in order get indexed on it and webmasters usually see this process as way to promoting their webpages, Normally it not necessary to do that because all major Search Engine like Google Search, Bing, Yahoo, AOL, etc use their bots (crawler or spiders) to find webpages from internet and index them to their database. So there only 2 reason why webmasters keep do submission to search engine :
Because most peoples start to use search engine to find information that why most webpages want to be listed on Search engine so they find the way to do that. There 2 methods used by webmasters until now, They can submit their homepage site in to search engine and leave the rest to bots index their entire pages inside, Or they can generate xml sitemaps that contain all url of their website. And because most search engine users not like to look over than one page results (SERPs), webmasters High eager to get the position in Top 10 in a search engine results.
To obtain good position in search engine they struggle to optimize their webpages and this process called as Search Engine Optimization (SEO), SEO covers many aspect like keywords density, webpages structure, web pages Loading Speed, Meta tags, Links Popularity, Page Rank and much more.
Unlike the Major Search Engine which possessed with ability auto detect and discover new webpages by sending bots to crawling them, many small search engine like Baidu.com in China is most popular while Google in second place. Without Submission services to these search engine your webpages may miss millions of visitors who might come from this Baidu.com.