Because pinging servers make the SEO a faster job. With the WordPress integrated configuration for “Update Services” (pinging) which you access via /wp-admin/options-writing.php?settings-updated=true

http://rpc.pingomatic.com
http://rpc.twingly.com
http://api.feedster.com/ping
http://api.moreover.com/RPC2
http://api.moreover.com/ping
http://www.blogdigger.com/RPC2
http://www.blogshares.com/rpc.php
http://www.blogsnow.com/ping
http://www.blogstreet.com/xrbin/xmlrpc.cgi
http://bulkfeeds.net/rpc
http://www.newsisfree.com/xmlrpctest.php
http://ping.blo.gs/http://ping.blo.gs/
http://ping.feedburner.com/
http://ping.syndic8.com/xmlrpc.php
http://ping.syndic8.com/xmlrpc.php
http://ping.syndic8.com/xmlrpc.php
http://ping.weblogalot.com/rpc.php
http://ping.weblogalot.com/rpc.php
http://feedburner.com/fb/a/pingSubmit?bloglink=http%3A%2F%2www.yourdomain.com/
http://ping.blo.gs/?name=example&url=http://example.com/
http://audiorpc.weblogs.com/RPC2
http://bing.com/webmaster/ping.aspx
http://blog.with2.net/ping.php
http://blog.youdao.com/ping/RPC2
http://blogping.unidatum.com/RPC2
http://blogsearch.google.com/ping/RPC2
http://catapings.com/ping.php
http://effbot.org/rpc/ping.cgi
http://news2paper.com/ping
http://ping.bitacoras.com/
http://ping.bloggers.jp/rpc/
http://ping.fc2.com
http://ping.groove-cast.com/ping.php
http://ping.pubsub.com/ping
http://ping.rss.drecom.jp
http://ping.syndic8.com/xmlrpc.php
http://ping.wordblog.de
http://rpc.bloggerei.de/ping/
http://rpc.pingomatic.com/
http://rpc.twingly.com/
http://rpc.weblogs.com/RPC2
http://serenebach.net/rep.cgi
http://services.newsgator.com/ngws/xmlrpcping.aspx
http://wasalive.com/ping/
http://xianguo.com/xmlrpc/ping.php
http://xping.pubsub.com/ping/
http://rpc.blogrolling.com/pinger/
http://rpc.technorati.com/rpc/ping
http://rpc.weblogs.com/RPC2
http://www.feedsubmitter.com
http://blo.gs/ping.php
http://www.pingerati.net
http://www.pingmyblog.com
http://geourl.org/ping
http://ipings.com
http://www.weblogalot.com/ping

/wp-admin/options-writing.php?settings-updated=true