Pinging just alerts those services that accept them a notification that you have new or updated content and the service will send it's spider back to crawl to take a look. That said, if you are regularly updating your site by adding new content then your site will be crawled anyway. SEO's use pinging to ping the pages where they placed backlinks on other sites where they have no control on how often it is updated or crawled.
Lets say you posted a comment with a backlink to your content on someone's blog on a post that has very few comments or is deep inside that persons blog. It might be a good idea to ping that post so that the spiders can see the new link pointing back to your content.
Pinging might get your backlink noticed but its relevancy is determined by how much juice it gets from where it is located.