Hi MonaM & welcome! PositionTech isn't the only company offering PFI to the three engines you mention, but is certainly one of the main places for PFI submission.
Submitting a URL via the PFI system will get that page listed quicker than would normally be the case and will have it reindexed every x number of days, so any new content is indexed pretty quickly and the new data shown in the search results. Those are the main benefits.
If you opt not to use PFI, your URL will most likely end up being listed in those engines eventually anyway, although that could take some time.
Using the site search [webmasterworld.com] should give you some useful info about the best ways to go about submitting your site, although you really should make sure the content is properly sorted out beforehand. Search Engine World [searchengineworld.com] also has some killer information about who provides results for who, who to submit to and how to go about it - as well as tools to aid in making the most of your pages.
R.
