pinging an Search Engine is same as pinging RSS directories, it informs that a new page or multiple new pages have been published and thus invites to revisit the site / RSS feed / sitemap-file. Thus the Search Engine and specially all RSS feed directories have created the ping service to faster find newest content and crawl faster, after a ping sent it typically takes seconds to minutes until the bot from that SE viists the URL submitted via ping
For example Google has a special ping service for blogs, so does Y for RSS and live plus dozens of RSS directories such as technorati, syndic8.
The result of ping to SE is that in happy situations you find your newest article already indexed/published in SE after 30-60 minutes following a ping, rarey a few horus.Some of the major SE revisit sitemap or RSS feeds on a regular basis multiple time per day/hr . thus a ping is no absolute must, but speeds up things.