This was pretty self-evident when Amari got here. Dak doesn't throw fades well, and he can't make a living out of always placing the ball where just his guy can get it (though he'll have several passes a game with just beautiful ball placement). To keep the chains moving, though, he needs guys who are where they're supposed to be when they're supposed to be, every time. And he needs guys who can get a ball anywhere in their catch radius because Dak consistently gets the ball in the area, but his WRs have to be able to slow down or reach back from time to time. Which is why Dak has a reputation for being inaccurate, when really he's throwing catchable balls, he's just not pin-point accurate.
This is why Dez was a bad fit for him, because he didn't have the best routes and he specialized in high pointing balls and end zone fades. It's why we're going to see a lot more crossing routes underneath coverage where Tony didn't like to throw those balls because they'd lead to tips and picks.
It makes sense that different guys have different preferences in terms of who to throw it to. Gallup, Cobb, Witten, and Cooper all do have those same skill sets Michael is talking about.