If you have a Klout account, you know that each day you are given a set number of K's which you can use to indicate topics other users are influential in. The giving and receiving of K's is an activity that generates a lot of user activity and keeps Klout users coming back daily. Now, LinkedIn is doing something along the same lines.
In a blog post, LinkedIn explained, "With just one click, you can now endorse your connections for a skill they’ve listed on their profile or recommend one they haven’t added yet."
Each time you endorse a skill for one of your connections they will receive a notification. It also generates activity for your profile that others can see.
It's too early to judge whether or not your connections will be impressed by your number of endorsements but initial opinions seemed to suggest that would not be the case.