The reason this is important is not just because it is sort of like the company is telling a star that they are giving them the ball, but also it allows a talent like Seth Rollins to get more attention. For example, if a former fan sees the game and Seth on the cover they maybe intrigue or if someone who watched Seth before he came to WWE sees Rollins on the cover they maybe more like to give it a chance. Basically, giving Rollins cover allows him to be in the eyes of casual fans just as much as regular fans which is something a "New star" has needed since Punk was on the cover of WWE 2013.
I for one am very happy that Rollins got the cover because I feel like it is WWE's way of saying that they aren't going to lean on the stars of the past as much anymore and may given guys like Rollins the chance they deserve. But like the WWE well have to wait and see what happens in the future.