Pinging your stores helps in announcing to the search engines and online directories that you have created or updated your store. The search engines (Google, MSN, Yahoo etc.) then send their automated spiders/robots to crawl your store and index the information they find onto their own sites.
It is recommended that you ping your store once after you have created it and anytime you update with a new article or product category. You can ping the store main page or any new page you create.