According to The Wrestling Observer, one of the main reasons Roman Reigns was involved in this year's Royal Rumble, despite already having a Universal Title match against Kevin Owens the same night, was to build his feud with The Undertaker for WrestleMania.
As for why Reigns entered the match at no.30, it's being reported that WWE wanted Randy Orton's win to get the best reaction possible from the live crowd. The WWE knew that putting Reigns at no.30, the number often used for big surprises or returns, would draw heat from the crowd and would get them behind Orton once it came down to them.
So in essence, the guy that WWE wants to be their top face is being used to draw heat so that instead of getting a lukewarm reaction, Orton could get one reserved for the hottest babyface in the company. What a mess.