You can bring the more traffic on your site, it give you opportunity to generate the quality traffic on your site.
Social Bookmarking site has huge link juice if your site is added here definitely it will transfer link juice to your site and help in ranking, according new google update you don’t need to search do follow and no follow sbm sites as google want natural backlinnk.
It’s my own experience, I thought, ranking depend on do follow backlink , but I was wrong , and started to build do follow backlinks in huge quantity after some time google banned site.
Xmarks is now dead, so in absence you can try these Xmarks Alternatives.
Social bookmarking helps in improving ranking of your keyword related to best web design services
Social Bookmarking is a Way to Improve the Ranking of Your Site or Blog