LinkedIn.com is in the business of selling ads (classified and otherwise). Much like other social networks they use many tactics to drive traffic back to their site and prompt as many page loads as possible to increase exposure to money-making features like job listings and recruiter contacts.
There has been much discussion about whether endorsements are useful to you, the user. Most commenters seem to distrust the endorsements of strangers or the unqualified. Some bloggers who have weighed in on the subject cite the thumbs-up rating approach as a cheapening of the data.
It seems to me this feature is more a gamification of the peer-review concept. Who doesn’t like to see more thumbs up, stars, tags, likes, or endorsements next to his or her name?