They are popping up at an ever-increasing rate. By socializing with others on the net through these sites, everything you join or participate in gives you more people to see your site. And, if you're selling stuff from your site, you'll want to go to the places (interest groups) on the social site where people are interested in what you're selling.
Not all web traffic is welcomed. Some companies offer advertising schemes that, in return for increased web traffic (visitors), pay for screen space on the site. There is also "fake traffic", which is bot traffic generated by a third party. This type of traffic can damage a website's reputation, its visibility on Google, and overall domain authority.