It does promotion, every user on eSteem sees your post on their feed and you get more visibility. Of course, it being worth or not depends on content and if it is able to attract more users to visit and read/interact. We will eventually release where users who read/interact with post earns ESTM from that promoted post... Additional benefit, esteem.app website visitors also gets to see those promoted posts, Surfer, Mobile and Website combined together gives good exposure. With time, this feature will be really competitive and we will make sure it serves in best interest of both promoter as well as consumer/reader.