1) You will have your site listed with the social bookmarking site. This way, when another user ads your site, it may already show up as an added site. So, the user can simply add it to their bookmarks or add to your submissions. Sometimes, when a site is initially submitted, it shows up that it is a new submission. So, overall, it is probably best that the first submission of your site is done by you.

2) When you add your site to a social bookmarking service, it usually shows up as a link on the site. As you know, getting links out there can help with your search engine popularity. So, its a good thing when you can get your website lin and description added to a popular site - for free

3) A lot of the social bookmarking services have search functionality or public listings of articles and links. So, to have a chance at your link showing up and receiving traffic - you need to have your website, link, and description published on the site. Here are some popular social bookmarking services to sign up with:

Digg This is a social news site. Users submit news stories and the more people that Digg (vote for) the story - the better chance the story will get prominent placement on Digg.com. You can submit your story, which, if nothing else, will get you a free link to your website
Yahoo Buzz This site is pretty similar to Digg. So you can submit your website as a story, and see if any free traffic comes your way.
Technorati This site has been called a search engine for blogs. To get your blog claimed (submitted) to Technorati - you need to sign up and place some code on your site. Also, as a registered user, you can add websites as your favorites which can get your link displayed on Technorati for free.
Delicious This is one of the original social bookmarking services. SO, you can sign up with the site and add as many bookmarks as you'd like. Make sure that these bookmarks are public, so that they can be shared with other web users. SO, you can easily have your link published and if you become an active user, you can share your bookmarks with others to try and grow your website traffic.
stumbleupon This is a pretty straight forward social bookmarking service. They ask you to download software when signing up, but you can become a member without downloading the software. Once you are signed up, you can add as many bookmarks as you like which allows you to get your link published on the site.
Spurl This site also wants you to do a quick download when you sign up, but you can bypass this step and get your links published as your bookmarks without downloading the software