With the exception of a user survey, time spent on a site is the best indicator that the user experience is an enjoyable and satisfactory one. Ultimately it's the combination of time spent on a site and returning visitors that truly speaks to an enjoyable user experience. The ability to measure stickiness allows companies to please customers to the highest degree possible. I like to use the analogy of clothes in a department store. If there's an item that's been sitting on a rack gathering dust for a couple of months, chances are that the item is going to get pulled.
Web sites work the same way.
A game that's not getting hits can be pulled and another put in its place. Or the opposite can occur. If a Kate Spade bag is selling like hotcakes, it will be reordered. A game getting lots of hits and return players will be updated regularly and used as a model to spin off successful variations. The whole point is to please customers, because when customers are pleased, advertisers are pleased.