I'm guessing it's just to come out and say thanks and goodbye (For now? Or forever?). Pretty lame that they wouldn't actually do this on Raw itself.
Highly doubt this is a tease to Sting vs Taker. Maybe we'll get a moment where Taker comes out to tip his hat to him or something, though. I'd take that as a definite sign this match will never happen, since they'd be cool with each other and have no issue.
It's starting to seem like Sting probably just wanted to wrestle at Mania and be done with it. If he and Taker are truly going to have a run in, I doubt they'd do it on the WWE Network instead of on Raw.