Ping service is for you to tell the world automatically that your blog was updated. Here is the definition of Ping from Wikipedia:
In blogging, ping is an XML-RPC-based push mechanism by which a weblog notifies a server that its content has been updated. An XML-RPC signal is sent to one or more "ping servers," which can then generate a list of blogs that have new material. Many blog authoring tools automatically ping one or more servers each time the blogger creates a new post or updates...