One-way backlinks:

By bookmarking your web site(s) to social bookmarking sites with a "do-follow" policy you create valuable on-topic one-way links back to your site.

Quick Indexing:

You can get your website indexed by all the major search engines by submitting them to the major social bookmarking sites (e.g. Digg, StumbleUpon, Delicious, etc.)

Traffic Generation:

You can get additional traffic from the social bookmarking sites by submitting quality content. Especially if your submission gets votes from other users traffic may increase substantially.

Personal Branding:

On most of the social bookmarking sites you will have a profile page that will help you establish your brand. This profile will also help you establish your authority in your field.