There are many answers but im going to talk about 3: free SEO, fast indexing & free publicity.

Free SEO – as we now know , Search engines like Google build relevance marks according to the incoming links, they analyize a few parameters: the site the incoming link is coming out of, the quality of the page, the quality of the link (anchor text, depth of the link, keyword relevancy to the page the link is coming out of and to the landing page) etc’.

What the search engines also calculate is the amount of the total number of links each site ha and the quality of the links and then give it a Page Rank (at least Google does…). What social bookmarking does is give site owners the opportunity to grow fast really easily.

For instance if you have good content and people “digg” it then they would post a link to you from digg, the same as for delicious , reddit and now from twitter too. These incredibly huge sites get crawled by search engines constantly which means that good sites that get talked about a lot on these 4 mega social sites get crawled and thus indexed on a more frequent basis.

If that’s all true (and it is) what this basically means is that you – the site owner, are getting free advertising from really simple publicity. If you just start bookmarking your site on these social sites and ask a couple of your friends to comment them / or book mark them as well than you would see an on going snow ball affect with your sites advertising, and all of that is for free.

Now the only hard part left is getting people to like your site and recommend it…