Without some sort of registration process, you don't have their contact details, and registration can be a long process that casual visitors are not interested in completing.
By offering visitors the ability to login using their existing Facebook / Google / Open ID / MySpace credentials, they connect to your site in a single click.
Needless to say, the mechanics behind this are complex. JanRain is a company that offers a single API to tie together all these disparate login systems.
What this brilliant module from Thakker does is allow you to set up a "unified" login for your Joomla site, using the (free) JanRain service.
It really is easy to do, taking a few seconds to install, and a minute or two to get a JanRain account and configure things.
Then a visitor can come to your site, and "login" even if they have not previously registered with a simple click.
If you do need support, I have found Thakker to be really excellent.