I will expose your URL's to a huge amount of web-crawlers/spiders and speed up the time of getting listed and indexed on search engines.
I do URL sub-mission in three categories:

Pseudo Search Engine,
XML-RPC Ping Service and
Pseudo Backlinks.

Pseudo Search Engine Sub-mission:
It enters your URL's in the respective search engine search bar.
Typically these entries get stored in an internal database and evaluated later on.
A direct URL entry increases your chance to get your site noticed and then crawled by a spider.

XML-RPC Ping Service Sub-mission:
Submits your URL directly to a wide list of blog listing services that keep track of the current web ecosphere.
An XML-RPC sub-mission increases your chance to get crawled and noticed by the important services, leading to a potentially faster indexing of your content.

Pseudo Backlink Sub-mission:
Creates simple lookup requests on internet service related websites like Info, Statistics, About or Whois domains about your URL.

After the sub-mission, an URL gets usually added to an internal DB that itself gets crawled in short intervals by search spiders.