First the constant Sabres bashing in your posts is as I've already noted, ***** annoying. It's in every thing you post regardless of the topic.
Second, why would Dallas trade Robertson? Because they don't think they can sign him next year and believe they can trade him and still get better. Will they? Idk, sounds like probably not but they might think they can use that 7.75 million this year and 10 million every year after for players that will help them win playoff games. Colorado traded Rantanen, guess they are at the children's table. Calgary traded Tkachuk, children's table.
Teams trade for players sometimes.