WWE decided to bring back The Undertaker at WWE Battleground for a few reasons. The main reason is because they wanted to give fans a hook to subscribe to the WWE Network. They were not unhappy about word leaking out before the show about Taker's return because the feeling was that it would generate a buzz for the show and get people to order. There was also a feeling that fans would see all of the talk about Undertaker's return on social media and the would subscribe so they could watch the Battleground replay. Obviously, they want to get their numbers up before they announce their updated subscriber count to their stock holders later this month.
Also, Undertaker's return should help the Raw ratings. The ratings have been disappointing as of late and Vince McMahon is known to react when ratings drop so Undertaker gives a reason for people to come back to the show. They need to get fans hooked into the product before football season starts.
Also, as noted before, WWE wants to make SummerSlam a bigger deal than it has been in previous years. The idea is to continue the tradition every year and they'll be adding more meet and greets and Superstar appearances during the week of SummerSlam.